Unfortunately, since I don't actually own one of the pets, I couldn't get a screenshot for that particular costume. Not only that, but the RNG involved in even getting transformed is quite high.
It would be awesome if there was a spell to turn us into a black, broom-riding, hat-wearing cat, but alas. ;P
As with my other Look-a-Like posts, it's helpful if you have the following:
1.) The help of a friend
2.) Potion of Illusion and/or Ai-Li's Skymirror
3.) Misc items that transformed my character into fun things
Pet: Creepy Crate
Item: Box o' Tricks
Notes: The item can only be used in Shadowmoon Valley if on the quest "To Legion Hold". Using a skymirror on a person transformed (or yourself) will preserve the costume even if you leave the zone.
Pet: Mini Diablo
Ability: Trick (demon) (Only available during Hallow's End)
Notes: Friend and Ai-Li's Skymirror or Potion of Illusion required
The Trick costume is random, so it may take many attempts to actually get the Mini Diablo costume. Since it only lasts for 45 seconds, the Skymirror or potion is necessary.
Pet: Nordrassil Wisp
Item: Hallowed Wand - Wisp (Only available during Hallow's End) or Wisp Amulet
Pet: Vampiric Batling
Item: Hallowed Wand - Bat (Only available during Hallow's End)