Minecraft version 1.12.2
Forge-14.23.2.2611
GraveStone-1.12.2-Extended-1.3.0b4

Ticking entity

java.lang.ClassCastException: com.hackshop.ultimate_unicorn.mobs.EntityUnicorn cannot be cast to net.minecraft.entity.passive.EntityHorse
at nightkosh.gravestone_extended.item.corpse.HorseCorpseHelper.setNbt(HorseCorpseHelper.java:90)
at nightkosh.gravestone_extended.item.corpse.CorpseHelper.getCorpse(CorpseHelper.java:150)
at nightkosh.gravestone_extended.helper.GraveGenerationHelper.lambda$addMobsItemsHandlers$14(GraveGenerationHelper.java:54)
at nightkosh.gravestone_extended.helper.GraveGenerationHelper$$Lambda$1880/62901118.addItems(Unknown Source)
at nightkosh.gravestone.helper.GraveGenerationHelper.createHorseGrave(GraveGenerationHelper.java:240)
at nightkosh.gravestone.core.event.EventsHandler.onEntityLivingDeath(EventsHandler.java:83)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_1591_EventsHandler_onEntityLivingDeath_LivingDeathEvent.invoke(.dynamic)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:179)
at net.minecraftforge.common.ForgeHooks.onLivingDeath(ForgeHooks.java:585)
at net.minecraft.entity.EntityLivingBase.func_70645_a(EntityLivingBase.java:1159)
at net.minecraft.entity.passive.AbstractHorse.func_70645_a(AbstractHorse.java:612)
at net.minecraft.entity.passive.AbstractChestHorse.func_70645_a(SourceFile:80)
at net.minecraft.entity.EntityLivingBase.func_70097_a(EntityLivingBase.java:1014)
at net.minecraft.entity.passive.EntityAnimal.func_70097_a(SourceFile:66)
at net.minecraft.entity.passive.AbstractHorse.func_70097_a(AbstractHorse.java:236)
at net.minecraft.entity.EntityLivingBase.func_70030_z(EntityLivingBase.java:232)
at net.minecraft.entity.EntityLiving.func_70030_z(EntityLiving.java:208)
at net.minecraft.entity.Entity.func_70071_h_(Entity.java:388)
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:2087)
at net.minecraft.entity.EntityLiving.func_70071_h_(EntityLiving.java:295)
at net.minecraft.entity.passive.AbstractHorse.func_70071_h_(AbstractHorse.java:682)
at com.hackshop.ultimate_unicorn.mobs.EntityMagicalHorse.func_70071_h_(EntityMagicalHorse.java:1002)
at net.minecraft.world.World.func_72866_a(World.java:1990)
at net.minecraft.world.WorldServer.func_72866_a(WorldServer.java:831)
at net.minecraft.world.World.func_72870_g(World.java:1952)
at net.minecraft.world.World.func_72939_s(World.java:1756)
at net.minecraft.world.WorldServer.func_72939_s(WorldServer.java:612)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:765)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:666)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:185)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:524)
at java.lang.Thread.run(Thread.java:745)

https://pastebin.com/FGLvxLcw

Hello. Unfortunately I can't reproduce your problem. Any way I fixed the part of my code which was mentioned in your crash log, so it should help.
Also I added 3 new configuration options:

• CreateCorpsesForModdedNotVanillaDogs
• CreateCorpsesForModdedNotVanillaCats
• CreateCorpsesForModdedNotVanillaHorses

Please give me to know if you'll need additional fixes.

P.S. I'll provide link on a new version here when it will be released.

THANKS!!