Like Kairuku and Dimetrodons, Otters can provide insulation based on their damage status. They are, however, easier to tame and can be carried on shoulder opposed to Dimetrodons, with a smaller hitbox than both the other two while retaining a good insulation capacity, so it's possibly to easily gather as many as 12 around a single egg.
On the con side, their insulation AoE is quite small, so hatching multiple eggs might require a ton of them, wasting creature slots. They have a very low Food stat as well, moreover they are mammals and thus cannot have their genetics stored in the form of eggs inside fridges, so stay ready to feed them fish every 4-5 days.
Otters also act like structure storage units, so one can use them to carry as many Artifacts as it wants.
Level up Melee Damage for portable ACs and living scarfs, Weight for Artifact backpacks and Food as a security measure on breeding stock.
Ptters are good for getting stuff in the water like pearls, black Pearl's and other stuff sometimes. They are also good for holding artifacts, and for keeping you warm in the snow biome or in the cold.
They can carry multiple artifacts, even on official, and provide hypothermic insulation against the cold. Literally a must have for running artifacts if you plan to spam run boss fights for element. 10/10 would smash again.