in 1.7.8 after a minute or two animals just keep spawning until there are huge amounts of them in a custom biome if you select only animals as the entities to spawn in the biome
Well if you selected cows, chicken pigs, and sheep, this is exactly what happens if you select these mobs to spawn there so I am closing this as works as designed. Please reply if you did not mean this
What entities did you put in the entity spawn list?