There is a cluckshroom (mushroom chicken) entity in minecraft earth. https://minecraft.gamepedia.com/Minecraft_Wiki:Projects/Minecraft_Earth_Wiki/Cluckshroom

There is already a discontinued 1.14 mod with an example: https://www.curseforge.com/minecraft/mc-mods/earth-mobs

There is also another active mod: https://www.curseforge.com/minecraft/mc-mods/earths-animals

So the issue can be closed.