They're originally from a webcomic called Out-of-Placers. They are basically an original furry species, ie not based on an existing animal.
Their intelligence tends, on average, to be less than humans'. But that doesn't mean that some yinglets aren't smarter than some humans, if you get me. And their society is younger so they're overall less advanced. It's hinted that this is because they were basically uplifted from a critter called a Lesser Yinglet (looks like a Yinglet but smaller and has the mind of a regular animal).
Uh, let's see. They lay eggs even though they are mammals, and females are extremely rare in canon (though not, as you may have noticed, on cohost). To the point where a male Yinglet might live his entire life without ever seeeing a female. Fortunately for them, male Yinglets are very.... sexually compatible with one another.
All that aside, the webcomic is really good. As you might have picked up it's very heavy on worldbuilding. And there are other sentient species too that haven't caught on among the furries like the Yinglets have, but they're still super neat.