Advance warning: My interpretation of "mob" is going to be somewhat relaxed. (That is, I plan to cheat.)
Paladin + Mini Minotaur = Palomino, a rideable golden horse with silver mane and tail. When riding a Palomino, it's a cinch to stirrup some adventure.
Goblin Scout + Lost Girl = Girl Scout, who never gets lost 'cos she knows how to use a map and compass… and she's not afraid to ask for directions. Flings sharpened Thin Mints for 40 calories damage apiece. 30% chance to drop some cookies, 15% chance to drop Samoa cookies.
The Twins + Piranha = The Piranha Brothers, Doug and Dinsdale. Together they count as one mob. Can be located and defeated only with the assistance of Spiny Norman the (non-Moebian) hedgehog NPC.
Penguin + any Bat = Comic Book.
Demon Eyes + your opponent = Propaganda.
Finally, here's one with the formula "mob + craftable item of furniture = food item."
Bunny + Toilet = Hoppin' John (rice, black-eyed peas, and bacon). Grants the "Well Fed" buff, brings good luck and wealth for one game year if eaten on New Year's Day.
[edited to add one more]
Rockfish + Fallen Star = Rock Star. Combined with The Axe gives 75% chance of popular hit, 50% chance of critical hit. Critics are pickier than regular listeners.