Fanonly, the Caitians.
But given nothing about her background was ever revealed "on camera", Lt. M'Ress could be a felinoid alien, a gynoid, a cat genetically "uplifted" to sapience, or just a human really into severely radical body modifications. Actually, we can at least assume she is a biological lifeform since Harry Mudd's "love crystals" had an effect upon her similar to the conditions it triggered in humans.
Yes, Peter David's "Gateway" novels went into some detail, but if one ONLY referred to screen time material, the field is WIDE open to interpretation. But I'll admit I'm rather pedestrian. I agree she's a felinoid alien as most material states.