Well recently I've been looking for some inspiration to do a commission I promised to *EchinosNoSeimei and I ran into an amazing artist, the man works for GW and his marine studies are the best I've seen. I'm talking about ~MikhailSavier of course. If you haven't seen his go do that now! I referenced this piece from one of his.
Navigators build up mutations over time as they ply their trade. Warp exposure. If I'd have to hazard a guess, I'd say that's how she developed a tail, but it might just be a family trait or something.