Mobs despawning/escaping

When chunks reload animals are able to escape through 2 block wide walls, other animals seem to despawn, I had about 50 rabbits and several horses in fenced areas, and over time all my horses but one disappeared and all rabbits but 4, the horses and rabbits are not in the surrounding area, so I assume they despawned. 2 of my tamed dogs that were sitting also despawned.