There are two variants of Otters, you have regular Otters that are roughly the size of a Skeever and they run away when you get close to them. And then you have Wolf Otters that are even bigger, around the size of a wolf. Wolf-Otters are vicious and will attack you if you get too close to them, however they're not too deadly unless you have a pack of Wolf-Otters attacking you. They're found near waterway and the open northern coasts.

They like to eat fish and clams, sometimes you'll find a couple of pearls on them since they like to collect natural shiny things.
Their fur is very valuable and they're sought after by hunters and trappers, and they're a good source of meat.

(Updated for SE, added to levelled lists etc. If you use other mods like skytest, no animals report crimes then you may need to make a compatibility patch or ensure changes are carried forward into bash/merge patch). Unofficial Skyrim patch required. 

If you want to see more of Skinwalker21's upcoming mods, you can always visit their discord channel here! https://discord.gg/t8tnhHW. Original LE mod is here.

(No otters were harmed while taking the screenshots)