*What I expected to happen was...:
Horses should spawn.

*What actually happened was...:
Minecraft crashed.

Minecraft crashes on approaching a horse spawner with the SpawnData or SpawnPotentials NBT tags attached. This spawner did NOT have ActiveEffects NBT tags on it. (And therefore does not duplicate MC-16191) and had a properly configured SpawnData tag containing an auxiliary Id tag. (And therefore does not duplicate MC-16257) (also tested with only the original EntityId tag on the spawner)

Steps to Reproduce:
1. Create a spawner for EntityHorse
2. Add the SpawnData NBT tag.
3. Paste in data from a valid horse entity.
4. Load the chunk containing said spawner.

To the mod that reads this: Please include a list of all bugs you feel this duplicates, as I'm fairly sure this is a unique issue that needs to be addressed within the code.