Removed in Cata
Zul'Gurub is being removed as a raid, so the mounts and pets are going. You'll still have them if you have them. Polymorph: Turtle will still be available "through other means" (no details on that).
May Return In Future
"Although we have no current plans to introduce new ways to obtain the Zul'Gurub mounts, pets, and the tribute mounts, they may return should we find a place where they fit in the future." (Lylirra)
ZG mounts and pets will be available again someday, but you should absolutely farm the ZG pet to sell for massive amounts of gold after ZG is removed. I'm not joking, people will pay a fortune in the "I can't collect this anymore" panic, and I'm betting the pet will return some way or another and then they'll feel stupid.