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.
Intelligence[]
The Sundayan otters were naturally intelligent, having their own language which the Doctor's TARDIS could translate. They also made small stone tools which they used to sever a limb of the swamp monster.
Their intelligence was initially suppressed to the level of a non-sentient animal by the swamp monster, and in this state they were captured by the Sundayan colonists for experiments. These experiments stopped when the effects of the swamp monster's poisoning wore off, and the otters were discovered to be as intelligent as humans. Before it was known that they were intelligent to begin with, Candy Kane theorised that the otters were gaining intelligence "by osmosis" due to their exposure to humans. (PROSE: Wetworld)