Quote from Oddzball
So, everyone kept saying animals didnt respawn in 1.8, but thats not really true. They still respawn, just slowly. Just the other day I had a sheep spawn in my new 1.8 map, inside of a big brick walled off area. I think they are just limited on how many can exist in any given area.
I've also had a pair of chickens and a pig spawn inside my structures after creating a new 1.8 map when it was official. Prior to that, several in game days of time had passed, I had collected all the wild game for hundreds of meters and put them into my dungeon/farm.
I had a wall and fence surrounding the area the new creatures appeared in. As well as a noted count of how many animals were in the dungeon (none had escaped). They were definitely newly spawned.