It seems that custom passive mobs that should spawn in biomes with no grass don't spawn (while the ones that should spawns in normal biomes covered by grass do).
In the RaolCraft workspace you have, there are 3 passive mobs that don't spawn: the Poisonous Cow which is supposed to spawn in the Poisonous Forest, the Glowshroom Cow which is supposed to spawn in the Glowshroom Lands and the Snow Chicken which should spawn in the Eternal Cold/Ice Dimension. (The Poisonous Forest and the Glowshroom Lands are two Overworld biomes)
Issue comments
Thank you for spotting a bug on my mod, I thought I set it to grass XD
I think you should change the ground material texture to the dirt texture to prevent confusion
Anyway, I tried changing the material of the PoisonGrass and I still can't find the Poisonous Cow, while I can easily find another mob which has a littler spawn rate but spawns in biomes with vanilla grass
These mobs used to spawn before, I don't remember on which version but it was before MCreator 1.8.3
I am not sure why this is happening as we use default registration for the mob types. I would suggest you to set the PoisonGrass material to Grass and try to see if this works.