I have read that the red hyenas in Scarlet Monastery will be replaced with the mastiff model in Cataclysm. My question is, if I have already tamed one of the red hyenas pre-cata, would it also turn into a mastiff after the expansion? Thanks.
As to my knowledge about "rare" hunter pets I would have to say no. Previously when Blizzard has changed in game mobs that were tameable the already tamed mob or companion if you will kept it's original model.
This is just all in theory though. You never know what Blizzard decides to do next. I'm still hoping to get a pink color scheme for my druid. You never know!
I hope this answer helped you out.
Petopia says your red hyena will change into a dog. Several other pre-tamed pets will also change.
If you had one of these pets tamed when it changes, your tamed pet WILL change also.
Onslaught Bloodhound Hyena - Black Mastiff Dog
Scarlet Tracking Hound Hyena - Red Mastiff Dog