Sundayan otters were encountered by the Tenth Doctor and Martha Jones in 2108, on their homeworld of Sunday. The Doctor at first mistook them for beavers, because of their incisors, which appeared to be designed to chew through wood. However, xenozoologist Ty Benson opined that they were visually closer to otters.
Still, this designation was only approximate, as they weren't actually mammals. They were egg-layers, but somehow additionally dissimilar to mammals that even the label "monotreme" didn't quite fit. A further difference between them and Earth otters was that they had toe webbing, which made them semi-aquatic. They were also highly intelligent, having their own language which the Doctor's TARDIS could translate. However, that intelligence was at first suppressed to the level of regular animals by the swamp monster, and they were taken in by the Sundayans for experiments. These experiments stopped when the otters were discovered to be as intelligent as humans. (PROSE: Wetworld)