OK sure, scratch the "moon" part, make it a planet.
* I deliberately used the "prehensile" bit ( def. being "chiefly an animal’s limb or tail- capable of grasping") cause it gives a bit more leeway when imagining the creature, it only has to be flexible enough to use tools so it can look whatever one imagines it.
*As for the size I intended to put that in but it slipped my mind later on, I imagined them to be some 3m tall, and as I pointed out at the end their superior stamin should have given them an edge which would counter the low birth-rate since if you can run for long enough and fast enough your average predator will look elsewhere for an easier prey. Also I could probably add a bit which would say something about their young being in a sort of a community crèche guarded by e.g. neuters.
Visten have always been most comfortable within the watery confinements of their home sea but have adapted and evolved into amphibians as the time progressed and their intelligence increased.
*I think this bit from the PHYSICAL DESCRIPTION part answers the question.
As for an opening sentence describing them, how about something along the lines of:
"Looks like a seahorse but flys those space fighters like a hawk"...
Oh and as for a type of planet most suited for them I'd say Earth would work fine cause it has plenty of water but still enough dry land for constructing a few cities and exploiting the land. Maybe an archipelago-like planet, but def. not an ocean world. Well, not anymore...