I am currently trying to change the spawn conditions for AnimalEntities, but so far have not been successful. I would like to change the spawn conditions of mobs that are considered AnimalEntities so that they can also spawn on Blocks in a custom Grass block tag, rather than only on vanilla grass blocks. We currently add no custom mobs, so currently the issue mainly resides in changing the vanilla mobs to these new conditions.

I have attempted using the LivingSpawnEvent.CheckSpawns event, but it appears that it doesn't trigger for AnimalEntities unless I used it incorrectly. If anyone could provide any help on this topic, it would be greatly appreciated.

It triggers but only when the spawn placement is valid. Since there is no sane way of replacing the spawn placements, you most likely cannot do this unless you build your own spawner currently. However, I would say this would be a valid candidate for a PR to expand the conditions to an optional tag.