We all know of the Classic Xenos or human-spawned Xenos, we've also seen dog Xenos, Predator Xenos, and we can only speculate the Jockey/Engineer Xenos or Ultramorphs. They'd have to be creatures that can be implanted by the Facehugger, like someone pointed out it would have to be Xenos that are not too small or too big for the Facehugger.
Kenner had those toys, but those were just for toy sales. But they do have good points, for some. Panthers, rhinos, bulls, gorillas, and snakes. But what else can they spawn from? I like the idea of a flying Xeno, maybe a condor.
I designed some of my own Xenos, except the dog, and panther is just the dog with a different color. These are the ones I put together, they're the best I can do.
And of course we got non-earth creatures they could spawn from, like the Pred-Hounds, River Ghosts, the Bugs the Marines mentioned, and all the different animals around the universe. I think a cancelled Aliens game actually had an idea like that with Xenos spawning from indigenous creatures.
The Deacon and the Newborn don't count because they're more like a genetic byproducts.