Dimetrosaurs are fun, with their poison AoE attack. Great for clearing low to mid levels of the classic champs.
Dragon Turtle eggs are a reward for the Turtle champ (1 egg dropped per spawn), but only the ones that hatch as 4-slotters after taming are worth keeping, as you can train them 1 level to maximize their resists and stats (plus maybe add some skills)
Not had a chance to tame much else, but some people are fond of the cats (if you can get one with healing) and the najasaurus