There are problems with the mod.
I think the mod is trying to create a variant of tropical fish, but it fails and an error occurs. It would be cool if there was an opportunity to configure: for which mobs Variant will be used, and for which it will be ignored.

Description: Rendering entity in world

java.lang.NullPointerException: Cannot invoke "net.minecraft.class_583.method_2816(net.minecraft.class_1297, float, float, float)" because "entityModel" is null
at dev.draylar.variant.client.VariantOverlayFeatureRenderer.method_4199(VariantOverlayFeatureRenderer.java:57)
at net.minecraft.class_922.method_4054(class_922.java:145)
at net.minecraft.class_927.method_4072(class_927.java:45)
at net.minecraft.class_959.method_4140(class_959.java:38)
at net.minecraft.class_959.method_3936(class_959.java:15)
at net.minecraft.class_898.method_3954(class_898.java:135)
at net.minecraft.class_761.method_22977(class_761.java:1563)
at net.minecraft.class_761.invokeRenderEntity(class_761.java)
at net.coderbot.iris.pipeline.ShadowRenderer.renderEntities(ShadowRenderer.java:582)
at net.coderbot.iris.pipeline.ShadowRenderer.renderShadows(ShadowRenderer.java:463)
at net.coderbot.iris.pipeline.newshader.NewWorldRenderingPipeline.renderShadows(NewWorldRenderingPipeline.java:629)
at net.minecraft.class_761.handler$ckp000$iris$renderTerrainShadows(class_761.java:10967)
at net.minecraft.class_761.method_22710(class_761.java:1235)
at net.minecraft.class_757.method_3188(class_757.java:1034)
at net.minecraft.class_757.method_3192(class_757.java:813)
at net.minecraft.class_310.method_1523(class_310.java:1122)
at net.minecraft.class_310.method_1514(class_310.java:737)
at net.minecraft.client.main.Main.main(Main.java:236)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

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

-- Head --
Thread: Render thread
Stacktrace:
at dev.draylar.variant.client.VariantOverlayFeatureRenderer.method_4199(VariantOverlayFeatureRenderer.java:57)
at net.minecraft.class_922.method_4054(class_922.java:145)
at net.minecraft.class_927.method_4072(class_927.java:45)
at net.minecraft.class_959.method_4140(class_959.java:38)
at net.minecraft.class_959.method_3936(class_959.java:15)
at net.minecraft.class_898.method_3954(class_898.java:135)
at net.minecraft.class_761.method_22977(class_761.java:1563)
at net.minecraft.class_761.invokeRenderEntity(class_761.java)
at net.coderbot.iris.pipeline.ShadowRenderer.renderEntities(ShadowRenderer.java:582)
at net.coderbot.iris.pipeline.ShadowRenderer.renderShadows(ShadowRenderer.java:463)
at net.coderbot.iris.pipeline.newshader.NewWorldRenderingPipeline.renderShadows(NewWorldRenderingPipeline.java:629)
at net.minecraft.class_761.handler$ckp000$iris$renderTerrainShadows(class_761.java:10967)
at net.minecraft.class_761.method_22710(class_761.java:1235)
at net.minecraft.class_757.method_3188(class_757.java:1034)

-- Entity being rendered --
Details:
Entity Type: minecraft:tropical_fish (net.minecraft.class_1474)
Entity ID: 167
Entity Name: Тропическая рыба
Entity's Exact location: 375.52, 56.83, 334.61
Entity's Block location: World: (375,56,334), Section: (at 7,8,14 in 23,3,20; chunk contains blocks 368,-64,320 to 383,319,335), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,-64,0 to 511,319,511)
Entity's Momentum: 0.01, 0.00, 0.00
Entity's Passengers: []
Entity's Vehicle: null
Stacktrace:
at net.minecraft.class_898.method_3954(class_898.java:135)
at net.minecraft.class_761.method_22977(class_761.java:1563)
at net.minecraft.class_761.invokeRenderEntity(class_761.java)
at net.coderbot.iris.pipeline.ShadowRenderer.renderEntities(ShadowRenderer.java:582)
at net.coderbot.iris.pipeline.ShadowRenderer.renderShadows(ShadowRenderer.java:463)
at net.coderbot.iris.pipeline.newshader.NewWorldRenderingPipeline.renderShadows(NewWorldRenderingPipeline.java:629)
at net.minecraft.class_761.handler$ckp000$iris$renderTerrainShadows(class_761.java:10967)
at net.minecraft.class_761.method_22710(class_761.java:1235)
at net.minecraft.class_757.method_3188(class_757.java:1034)
at net.minecraft.class_757.method_3192(class_757.java:813)
at net.minecraft.class_310.method_1523(class_310.java:1122)
at net.minecraft.class_310.method_1514(class_310.java:737)
at net.minecraft.client.main.Main.main(Main.java:236)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

-- Renderer details --
Details:
Assigned renderer: net.minecraft.class_959@17bc6c99
Location: 27.87,-6.87,8.23 - World: (27,-7,8), Section: (at 11,9,8 in 1,-1,0; chunk contains blocks 16,-64,0 to 31,319,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,-64,0 to 511,319,511)
Rotation: -47.8125
Delta: 0.09998894
Stacktrace:
at net.minecraft.class_898.method_3954(class_898.java:135)
at net.minecraft.class_761.method_22977(class_761.java:1563)
at net.minecraft.class_761.invokeRenderEntity(class_761.java)
at net.coderbot.iris.pipeline.ShadowRenderer.renderEntities(ShadowRenderer.java:582)
at net.coderbot.iris.pipeline.ShadowRenderer.renderShadows(ShadowRenderer.java:463)
at net.coderbot.iris.pipeline.newshader.NewWorldRenderingPipeline.renderShadows(NewWorldRenderingPipeline.java:629)
at net.minecraft.class_761.handler$ckp000$iris$renderTerrainShadows(class_761.java:10967)
at net.minecraft.class_761.method_22710(class_761.java:1235)
at net.minecraft.class_757.method_3188(class_757.java:1034)
at net.minecraft.class_757.method_3192(class_757.java:813)
at net.minecraft.class_310.method_1523(class_310.java:1122)
at net.minecraft.class_310.method_1514(class_310.java:737)
at net.minecraft.client.main.Main.main(Main.java:236)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)