What kind of aquatic animal is Tarka according to the title of a novel by Henry Williamson?

Tarka the Otter: His Joyful Water-Life and Death in the Country of the Two Rivers is a highly influential novel by Henry Williamson, first published in 1927 by G.P. Putnam's Sons with an introduction by the Hon. Sir John Fortescue. It won the Hawthornden Prize in 1928 and remains Willamson's best-known and most popular work, having never been out of print since first publication.

As its title suggests, the novel describes the life of an otter, along with a detailed observation of its habitat in the country of the River Taw and River Torridge in North Devon (the "Two Rivers"); the name "Tarka" is said by Williamson to mean "Wandering as Water" (p. 10). Though often now characterized as a children's book, Tarka has influenced literary figures as diverse as Ted Hughes and Rachel Carson.