Animals all carry the same treasure: none. So pickpocket them before killing them if you want to get anything. If you kill them with arrows, you can still retrieve your arrows as long as it took more than one to kill it. On single-shot kills, you will find an empty inventory screen.
What can you say about flying rodents and crawling rodents? They dont do much damage and dont have many hit points, but they do carry diseases (about 0.5% chance per successful hit).
Pretty easy to kill. They hit slowly, though, so you can probably finish them off before they get in more than one or two swings. Once you've got a few hit points under your belt, they're excellent for practicing Hand-to-Hand.
Theyre slow hitters, but their poison paralyzes if you fail your save. Surprisingly enough the paralyze counts as a spell, so it can be reflected or absorbed. If paralyzation isn't a worry, practice your Hand-to-Hand on them.
SLAUGHTERFISH: (Armor 6, Health 15 to 50, Damage 2-12) Underwater only. They bite frequently. If you have soul-bound one of these into an item, they can and do appear above ground when the item breaks -- makes for humorous encounters.