If you have an oil-stained wolf or any other "glitch tamed" pet with a visual effect, you probably don't want to risk summoning it in a Timewalker dungeon.
http://us.battle.net/wow/en/forum/topic ... ?page=4#72
As we all know, if you bring one of these pets into arena they lose their special appearance forever. A similar system may be in place for timewalking. Here is the relevant bit:
Be careful!Most buffs are cleared when entering Timewalking, similar to Arena. This is necessary to ensure that the down-scaling that occurs when you enter the dungeon is done cleanly.
EDIT:
It's obvious at this point, from people who have lost their treasured pets, that this is confirmed.
Do not take these pets into Timewalking dungeons.