As it happens, one of the "deep space life forms" featured in the SFU is the
Space Manta; it's featured
on the cover (and in the cover story) of
Captain's Log #40.
And there are Space Dragons as "living monsters" in the SFU; indeed, a reptilian species in the
Omega Octant, the Branthodons, captured a Space Dragon hatchling and used it to clone their own Dragonships. (Each Dragonship has a series of organic and cybernetic "exoskeleton" hull segments grafted onto their back plates; while the brains of the Dragonships themselves are lobotomized so as to try and prevent them from "going wild".)
There is little info available about the "Old Kings"; even the name is a placeholder, since no-one knows what they called themselves. There is some speculation of them having had the stereotypical "grey alien" look. So far, none of their ships have been shown directly; by the time of the earliest Klingon ships currently in print for
SFB (the "warp-refitted" upgrades to the D3 and F3), the Empire had long since shifted to the "manta ray" design layout.
By and large, the Old Kings seemingly treated the Klingons and other subject species reasonably well, albeit at something of a distance. (The Klingons introduced security stations onto their ships in response to the Vergarian revolt, but there may or may not have been similar stations watching their ancestors who served on Old King starships.) The Klingons were fortunate in that the Old King derelicts were left relatively close by for them to grasp a hold of; no-one is quite sure if this was intentional (to give the Klingons a head-start), or merely an unintended by-product of the Old Kings' sudden departure from known space.
Actually, the Old Kings' realm also covered much of what would become Federation space, but they seemingly chose not to contact (or recruit) any pre-warp species there. They did have a series of
very limited contacts with Vulcan, via an outpost in the Zeta Reticuli system; in the "modern" SFU era, the planet which hosted this Old King outpost is under "research quarantine".
(There are three other sets of "Kings" - for lack of a better term - in the pre-history of the Alpha Octant. The "Leopard Kings" apparently seeded the ancestors of the Kzintis, Lyrans, Carnivons, and other related species onto their respective home planets. The "Lizard Kings" seemingly did something similar with the ancestors of the Gorns and Paravians. While the "Spirit Kings" uplifted the Hydran species into space, taking a much more "hands-on" approach relative to how the Old Kings dealt with the Klingons.)