These are awesome shoulder pets, as mentioned, they keep you warm and the weight of anything in its inventory is halved but they also make great bodyguards, I put a little into weight and health the rest into melee, not only will it increase it's insulation buff but if set to attack target, will bite anything that is aggressive and gets too close, Ollie has saved my ass a few times now.

Also, to tame one I would suggest using a flying mount to pick it up and drop it in a 1x1 with a 1 door for access and bring an Ichthyornis for collecting the fish, set the ichthy to agressive and follow, stand next to the door and wait for the fish to be delivered to feed the otter.

If you want to keep it alive as long as possible, equip it with a ghillie mask as it will get the camouflage buff as well, this will help it stay hidden if accidentally dropped.