True. Since lycanthropy affects humanoids regardless of whether or not they resemble birds, reptiles, amphibians or mammals, you could still have Dragonborn lycanthropes whose hybrid or animal forms may or may not have something draconic about them. They'll just be lycanthropes with a breath weapon.
"Is it a Winter Wolf or is it a Dragonborn (Silver or White) Werewolf? 
If you aren't going to limit lycanthropy to mammals, doesn't the real question become "what does a plasmoid werewolf look like?"True. Since lycanthropy affects humanoids regardless of whether or not they resemble birds, reptiles, amphibians or mammals, you could still have Dragonborn lycanthropes whose hybrid or animal forms may or may not have something draconic about them. They'll just be lycanthropes with a breath weapon.
Sure, though it would be a little weird for them to not come from one side or the other. After all, the ancestors of modern mammals--the synapsids--were quite similar to reptiles despite not being very closely related. It took a long time, fifty million years or more, for features like hair to develop.
How about a changeling?Except that Plasmoids have the Ooze type, and therefore are immune to lycanthropy.
I don't see why, even if you limit it to vertebrates, there are plenty that are neither mammal nor reptile, such as birds/dinosaurs. And Thri-Kreen and Plasmoid PCs aren't even vertebrates.
Birds (and dinosaurs) are part of sauropsida--which includes all modern reptiles. Hence why I made the distinction I did. It would be strange for a reptile-like creature to not belong to one of those two clades, particularly when dragonborn are quite clearly amniota, as they lay hard-shelled eggs like both sauropsida and (pre-viparous) synapsida but unlike any other clade of organism.
There is no reason they would belong to any terrestrial group. Dragons don't come from Earth.
