This "Non-Combat Pet Review" is made for Molten-WoW,
but supports all 3.3.5 WoW servers (private).
This is not actually the pet, but an item that allows you to learn how to summon and dismiss the Mini Diablo mini-pet (it is a "non-combat pet", meaning that it will not aid you in combat).
On retail (Blizzard servers/realms), it is only obtainable by purchasing the original Collector's Edition of World of Warcraft.
ON MOLTEN-WOW, it is available in the Points Shop for 5 vote points.
NOTE: It can NOT be obtained the "normal way".
Mini Diablo is one of the most wanted/"rare" mini-pets in WoW. On Molten-WoW however, it's popularity is based on it's looks only - and oh boy is it popular. It's "dark feeling" is something that goes along with almost any character in-game, especially the ones of the faction called Horde.
"Mini Diablo is inspired by the title villain of Blizzard's Diablo series of games (the name is literal; it's a very tiny version of the Lord of Terror). As its idle animation, it will occasionally breathe fire." - WoWWiki