There's a lot of depends in the answer:
If you have 100 bushido, then it's a lesser hiryu.
If you're a miner, then it's a fire beetle.
If you want to carry a lot of loot, then it's a blue beetle.
If you want damage reduction, then it's a swamp dragon (with player crafted barding)
If you're a necromancer, then you can't go wrong with bake kitsune (but you can't bond them without taming skill)
If you want to collect demon bones or do IDOCs, then pack horses are pretty good there.
Then you can always do the typical super-chicken, trained squirrel, trained cow oddness, but they won't help much.
