Sundayan otter

Sundayan otters were encountered by the Tenth Doctor and Martha in 2108 on their homeworld. 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. (NSA: Wetworld)