I have a fairly long list of mobs, heavily influenced by Mo' Creatures, that I would love to see added to the game. The mobs (and, ideally, the associated items) I would like to see are as follows; Elephants and Mammoths, with armor and chest sets for both, snails and insects, turkeys, horses (Maybe not all of them, but some, plus zebras, donkeys, mules and the special horses) and their armor, ostriches and their variants, scorpions and their variants, manta- and stingrays, jellyfish, turtles, scorpions and their variants, mice/rats, deer, fish, boars, bunnies, birds and foxes. Most of the mobs here would work well simply for aesthetic purposes, such as deer and fish, and some, I must admit, were suggested solely for the benefit of the world I'm currently building, as armored horses and tamed scorpions and mammoths would be fearsome mounts for any city to possess. Also, I apologize if listing multiple animals counts as "more than one idea."
A few of those mobs are hostile. The scorpions are similar to spiders, they are only hostile at night. Stingrays and Jellyfish are somewhat hostile, they don't actively pursue you, but they sting if you get close. And "fish" includes piranhas. You aren't really supposed to ask for mods, but this list is based on Mo' Creatures, so look that up for details.