I was trying to inspect the entiity data for a skeleton that I had spawned using a spawn egg. I used this command:

/entitydata @e[type=Skeleton] {}

The result was that the game said "Entity cannot be found".
I also tried /testfor @e. Again, the skeleton did not show up using this.

I repeated this with cows, mooshrooms, zombies, and many others and it seems that all mobs spawned in with spawn eggs are somehow 'nonexistant' in terms of trying to target them or locate them with a command. Entities summoned using the /summon command work normally, but those made with spawn eggs do not.