I am using a few sometime soon. What you've said is about all I know. They also come in a really weird CR band of 5, 11-13. Nothing higher, nothing lower. There is no unifying biology. It's like someone said "I want to make a fox wizard and an evil Grey and an intelligent mosquito. Let's put them all on the same team!"
If there's more good info on them I'd like to know.
Since nobody has really given you the lore on them, I will do my best. Note that I am not an expert on Planescape, which is where I feel the most and best lore regarding them comes from. I am sure that what I am posting will be incomplete and will likely have some errors in it.
Yugoloths are the incarnation of Neutral Evil, as devils are Lawful Evil and demons, Chaotic Evil. They are quite mercenary; they hire themselves out to both sides in the Blood War, but almost inevitably betray their employers when a better offer comes along. They are from the NE planes of Hades and Gehenna, and appear also on their neighboring planes. According to Planescape lore, one might say that the upper echelons of yugoloths are obsessed with determining the true nature of evil, and whether LE or CE is closer to being pure evil or, looked at another way, a better path to evil.
The 'loths trace their origin to creatures known as the baernoloths- primeval antecedents of the yugoloths (and maybe most of the other fiends) who maybe created the rest? It's fairly certain that they (or at least one of them) are at least responsible for the creation of the demodands/gehereleths, yet another branch of non-demon, non-devil fiend (the faratsu, kelubar, and shator). These are shunned even by other fiends.
Yugoloths are known for Byzantine plots spanning eons, being deeply manipulative, and- as I said- being obsessed with determining the true nature of evil. There are a number of types other than those in the MM; some appear in the Mordenkainen's Multiverse of Monsters book. There might be a few others. Also, I think there are some from an old Dragon Magazine article that masquerade as weapons that fall into the hands of mortals in order to manipulate them into evil. (Memories hazy, it has been well over a decade since I read that article.)
There are also another group of yugoloths, the guardian yugoloths, which have not appeared since 2e as far as I know; these are usually found bound by mortal spellcasters as guardians, and IIRC are sometimes depicted as neutral in alignment. I seem to recall some lore about them being kind of like the golems of the 'loth world- maybe not true yugoloths at all, but instead creations of the 'loths.
I hope this helps a little bit!