The title states Minecraft version & forge version & optifine version.

the beta version currently in use is untamedwilds-1.16.5-1.5.8.jar
( link to it is below )
https://www.curseforge.com/minecraft/mc-mods/untamedwilds/files/3877295

Here's the stack trace.

---- Minecraft Crash Report ----
// I let you down. Sorry :(

Time: 8/16/23 2:51 PM
Description: Rendering entity in world

java.lang.NullPointerException: Rendering entity in world
at untamedwilds.util.EntityUtils.getSkinFromEntity(EntityUtils.java:384) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.entity.ComplexMob.getTexture(ComplexMob.java:295) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.client.render.RendererTortoise.getEntityTexture(RendererTortoise.java:30) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.client.render.RendererTortoise.func_110775_a(RendererTortoise.java:13) ~[?:1.16.5-1.5.8] {re:classloading}
at net.minecraft.client.renderer.entity.LivingRenderer.func_230496_a_(LivingRenderer.java:226) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:cavesandcliffs.mixins.json:client.LivingEntityRendererMixin,pl:mixin:APP:twoplayersonehorse.mixins.json:LivingRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.entity.LivingRenderer.func_225623_a_(LivingRenderer.java:178) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:cavesandcliffs.mixins.json:client.LivingEntityRendererMixin,pl:mixin:APP:twoplayersonehorse.mixins.json:LivingRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.entity.MobRenderer.func_225623_a_(MobRenderer.java:47) ~[?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.client.renderer.entity.MobRenderer.func_225623_a_(MobRenderer.java:20) ~[?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.client.renderer.entity.EntityRendererManager.func_229084_a_(EntityRendererManager.java:311) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:mixin.imdlib-betteranimalsplus.json:EntityRenderDispatcherAccessor,pl:mixin:APP:byg.mixins.json:client.MixinEntityRendererManager,pl:mixin:A}
at net.minecraft.client.renderer.WorldRenderer.func_228418_a_(WorldRenderer.java:2120) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:citadel.mixins.json:client.WorldRendererMixin,pl:mixin:APP:cavesandcliffs.mixins.json:client.WorldRendererMixin,pl:mixin:APP:mixins.cofhcore.json:WorldRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.WorldRenderer.func_228426_a_(WorldRenderer.java:1742) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:citadel.mixins.json:client.WorldRendererMixin,pl:mixin:APP:cavesandcliffs.mixins.json:client.WorldRendererMixin,pl:mixin:APP:mixins.cofhcore.json:WorldRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.GameRenderer.func_228378_a_(GameRenderer.java:1022) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:mermod.mixins.json:GameRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:693) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:mermod.mixins.json:GameRendererMixin,pl:mixin:A}
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:977) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:chat_heads.mixins.json:MinecraftMixin,pl:mixin:APP:architectury.mixins.json:MixinMinecraft,pl:mixin:APP:mining_helmet.mixins.json:MinecraftMixin,pl:mixin:APP:abnormals_core.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:chat_heads.mixins.json:MinecraftMixin,pl:mixin:APP:architectury.mixins.json:MixinMinecraft,pl:mixin:APP:mining_helmet.mixins.json:MinecraftMixin,pl:mixin:APP:abnormals_core.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:37) [forge-1.16.5-36.2.39.jar:36.2] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$505/1658534033.call(Unknown Source) [forge-1.16.5-36.2.39.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?] {re:classloading}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?] {re:classloading}

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Render thread
Stacktrace:
at untamedwilds.util.EntityUtils.getSkinFromEntity(EntityUtils.java:384) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.entity.ComplexMob.getTexture(ComplexMob.java:295) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.client.render.RendererTortoise.getEntityTexture(RendererTortoise.java:30) ~[?:1.16.5-1.5.8] {re:classloading}
at untamedwilds.client.render.RendererTortoise.func_110775_a(RendererTortoise.java:13) ~[?:1.16.5-1.5.8] {re:classloading}
at net.minecraft.client.renderer.entity.LivingRenderer.func_230496_a_(LivingRenderer.java:226) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:cavesandcliffs.mixins.json:client.LivingEntityRendererMixin,pl:mixin:APP:twoplayersonehorse.mixins.json:LivingRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.entity.LivingRenderer.func_225623_a_(LivingRenderer.java:178) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:cavesandcliffs.mixins.json:client.LivingEntityRendererMixin,pl:mixin:APP:twoplayersonehorse.mixins.json:LivingRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.entity.MobRenderer.func_225623_a_(MobRenderer.java:47) ~[?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.client.renderer.entity.MobRenderer.func_225623_a_(MobRenderer.java:20) ~[?:?] {re:classloading,xf:OptiFine:default}
-- Entity being rendered --
Details:
Entity Type: untamedwilds:tortoise (untamedwilds.entity.reptile.EntityTortoise)
Entity ID: 8028
Entity Name: Tortoise
Entity's Exact location: -3827.70, 75.00, -500.63
Entity's Block location: World: (-3828,75,-501), Chunk: (at 12,4,11 in -240,-32; contains blocks -3840,0,-512 to -3825,255,-497), Region: (-8,-1; contains chunks -256,-32 to -225,-1, blocks -4096,0,-512 to -3585,255,-1)
Entity's Momentum: 0.00, -0.08, 0.00
Entity's Passengers: []
Entity's Vehicle: ERROR NullPointerException: null

-- Renderer details --
Details:
Assigned renderer: untamedwilds.client.render.RendererTortoise@42fd774
Location: 4.93,-4.00,0.23 - World: (4,-4,0), Chunk: (at 4,-1,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Rotation: 115.3125
Delta: 0.2600752
Stacktrace:
at net.minecraft.client.renderer.entity.EntityRendererManager.func_229084_a_(EntityRendererManager.java:311) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:mixin.imdlib-betteranimalsplus.json:EntityRenderDispatcherAccessor,pl:mixin:APP:byg.mixins.json:client.MixinEntityRendererManager,pl:mixin:A}
at net.minecraft.client.renderer.WorldRenderer.func_228418_a_(WorldRenderer.java:2120) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:citadel.mixins.json:client.WorldRendererMixin,pl:mixin:APP:cavesandcliffs.mixins.json:client.WorldRendererMixin,pl:mixin:APP:mixins.cofhcore.json:WorldRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.WorldRenderer.func_228426_a_(WorldRenderer.java:1742) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:citadel.mixins.json:client.WorldRendererMixin,pl:mixin:APP:cavesandcliffs.mixins.json:client.WorldRendererMixin,pl:mixin:APP:mixins.cofhcore.json:WorldRendererMixin,pl:mixin:A}
at net.minecraft.client.renderer.GameRenderer.func_228378_a_(GameRenderer.java:1022) ~[?:?] {re:mixin,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:mermod.mixins.json:GameRendererMixin,pl:mixin:A}

-- Affected level --
Details:
All players: 2 total; [ClientPlayerEntity['bmwchickk'/10021, l='ClientLevel', x=-3832.63, y=79.00, z=-500.85], RemoteClientPlayerEntity['Caithuwu'/371, l='ClientLevel', x=-3833.92, y=75.00, z=-508.16]]
Chunk stats: Client Chunk Cache: 729, 287
Level dimension: minecraft:overworld
Level spawn location: World: (8,64,8), Chunk: (at 8,4,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 1 game time, 1 day time
Server brand: forge
Server type: Non-integrated multiplayer server
Stacktrace:
at net.minecraft.client.world.ClientWorld.func_72914_a(ClientWorld.java:617) ~[?:?] {re:mixin,xf:OptiFine:default,re:classloading,xf:OptiFine:default,pl:mixin:APP:architectury.mixins.json:MixinClientLevel,pl:mixin:APP:abnormals_core.mixins.json:client.ClientWorldMixin,pl:mixin:A}
at net.minecraft.client.Minecraft.func_71396_d(Minecraft.java:2031) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:chat_heads.mixins.json:MinecraftMixin,pl:mixin:APP:architectury.mixins.json:MixinMinecraft,pl:mixin:APP:mining_helmet.mixins.json:MinecraftMixin,pl:mixin:APP:abnormals_core.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:623) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:chat_heads.mixins.json:MinecraftMixin,pl:mixin:APP:architectury.mixins.json:MixinMinecraft,pl:mixin:APP:mining_helmet.mixins.json:MinecraftMixin,pl:mixin:APP:abnormals_core.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:37) [forge-1.16.5-36.2.39.jar:36.2] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$505/1658534033.call(Unknown Source) [forge-1.16.5-36.2.39.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?] {re:classloading}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?] {re:classloading}

Sorry, but I am not going to fix this one

Not supporting 1.16.5 any longer, and since this looks like OptiFine is doing OptiFine things in the background, I'm not going to touch this

Apparently, according to my friend, they had put 5 turtles in their house & logged off, & then, when they went to reconnect to the server, this crashing began for them. I had to go to their home and kill their turtles so they could re-log back on the server lol.