The Demon Breed - a "Hub" novel by James H. Schmitz (1968.) Serialized in Analog September & October 1968 as "The Tuvela." Here's the flap-copy from the first hardcover, which gives a reasonable summary or set-up:

Some seventy years ago the Parahuans had come out of space to launch simultaneous attacks against the Hub - their name for the dozen worlds of the humans. They were repulsed - and for seventy years they had been smarting in defeat, trying to understand how it could have happened. They had achieved perfection at all levels, including immortality. They were the Everliving. Yet, humans - an inferior race - had almost completely destroyed the Parahuan forces sent to occupy their planets. The experience stunned the Parahuans. It affronted logic - unless the humans had been helped... helped by some superior, some superhuman beings...

And so, the Parahuans evolved the Tuvela Theory - that a group of superhuman creatures called the Tuvelas were the secret rulers of the Hub, that it was the Tuvelas who must be beaten if the Parahuans were to destroy the Hub! To test the theory and discover the extent of the Tuvelas' power, the Parahuans dispatch a small force to the aquatic planet, Floatwood - where they run headlong into Nile Etiand, a beautiful young research scientist.

Nile is not superhuman, but she is an unusually resourceful human, and she has an equally resourceful ally - her seven-foot-long, talking, pet otter, Sweeting. Realizing that if she is to prevent the destruction of the Hub, she must convince the Parahuans that she possesses superhuman abilities, Nile conceives of a plan that calls for all the resources of Sweeting - and of Nile herself!...