My Hunter pet acquired a few costume buffs when doing one of the new events starting in Dalaran. There is no option to remove these, forcing to just wait it out for an hour.
Is there a macro command or some such to remove these buffs?
Is this a bug in the event where pets probably should not be getting costumed at all?
My pets have just been dropping the buffs, do not know how. Have you tried dismissing and resummoning?
I will say I found myself laughing uncontrollably at myself when I realized that this player that kept running after my tags and beating me was not a player at all but my pet 
Edit:
Just did Deviant Delights on this toon (BM Hunter), once again, both my pets had the disguise, mounting up and headed to Onyxia and my Main pet was no longer transformed, dismissed and still not transformed but my stabled pet (Talent) is still transformed.
Edit2: Scratch that, it’s my stabled pet that has dropped disguise, not my stabled pet.
Bug? I don’t know.
That same event will also transform the Warlock Felguard. Thankfully it does not affect my Wild Imps. I did not try my succubus yet, but if the Felguard can catch that transform, she probably will too. My Beast Master Hunter sees both pets transform. As far as I have noticed, the transform sticks around for the full duration even after I leave the instance.
Is there an addon that enables the removal of buffs on a pet?
I tried IceHUD but it only lets me view them.
There is no way to “remove” pet buffs/debuffs unless they are dispellable by a player. If Dismissing the pets and resummoning doesn’t work you just have to wait the duration. Not sure why this is such an issue, I think it was fun and hilarious having my warlock pets be pirate/ninja.
I too noticed my stabled pet dropped the disguise flying to Onyxia , but not my main pet. I think they still obey Kill Command, but their auto attack is reduced to a punch every few seconds.
Having a pet pirate is novel indeed.