Baseless Ylla Headcanon Thing
So, from the logbooks we know that the Ylla were peers of the Chozo, Luminoth, and Reptilicans, and not much more than that. Still, as an educated guess, I think it isn't too much of a stretch to assume their technology is compatible with Chozo stuff, they've been around for a while, and they probably have at least some amount of ability to tamper with genetics (like the Chozo) or heavy terraforming and climate control. (like the Luminoth and Reptilicans.) We haven't seen them directly, but that doesn't necessarily mean much given how remote most of the Metroid games have been, but going off the pattern of the other races, they're probably quite reclusive nowadays.
One other thing I like to think, is that since it's been hinted the Chozo weren't always so peaceful, and some darker groups may still be around somewhere, the Ylla have probably seen the Chozo at their worst. So while they eventually allied with them and shared technology, they're also probably among the few who still know about their more militaristic expansionist past, and so might be a bit more suspicious of Chozo influence than most; even if not hostile to Samus, they'd probably be quite wary of her regardless.
Another idea I like, is that since we have lizard aliens, moth aliens, bird aliens, etc., the Ylla would probably keep to that theme in some fashion; I have gone with the idea of them as being quite bat-like, kinda like this. I also had this idea of them being somewhat foils to the Luminoth, favoring darkness rather than light; the races probably got along amicably enough, but the fundamental culture difference probably took a while to overcome.
From this idea of them favoring darkness, I had the idea that their planet of origin is an extremely remote place, at the very edge of the solar system it's in (if it's not a rogue altogether) and so recieves very little sunlight. It has lots of moons that produce strong tidal geothermal activity, and a dense atmosphere to keep the heat in. (Said atmosphere also means they can probably still glide despite being of humanoid size, so their wings can still function as such, instead of becoming vegistal arms like the Chozo. This would also make them retain a fairly spindly, lightweight frame; much better suited to stealth than direct confrontation. Because of the geothermal activity, they still evolved to have normal eyesight, (to steer clear of glowing lava flows and whatnot) but it's still fairly weak, and sensitive to brighter lights; sound would still be really important to them, and they'd probably have a lot of technology involving sonics. (Possibly some parallel to the Annihilator Beam, but without the convoluted matter/antimatter shenanigans.) Also, they, and probably most other creatures on their planet, would have developed bioluminescence as a means to find one another visually in the dark, foggy environments. Still, light in general is off-putting to them rather than welcoming; while most cultures in general tend to have some amount of appreciation for their suns, the Ylla very much do not; combined with the danger association from volcanic activity, there's a lot of culture clash with other races. For them, saying someone 'shines brightly' would be an insult rather than a compliment.
Once they became spacefaring they probably attempted to terraform other worlds, but due to most habitable locations being closer in to their respective suns and stuff, it would take a lot more effort to reproduce the unique conditions of their homeworld. Between that and encountering the early Chozo still steamrolling the rest of the galaxy to their needs, they probably retained a fairly limited presence in general, and like the Luminoth, potentially decided to just stick to one planet altogether. They probably also aren't that involved with the Federation either as a result, preferring to be left alone rather than get thrust into major galactic politics again. Still, that they are more aware of the Chozo's darker side, would mean any hostile groups still around and still harboring greater ambitions, would want to target the Ylla quickly to prevent them from warning the rest of the galaxy...
(Sorry for the giant text-wall here. I've put waaay too much thought into this concept, still debating possibly trying to do something with it.)
