This has got to be one of the weirdest creatures in D&D.
I had a character encounter one once and I was not pleased! They are a pain in the rump and perhaps just as treacherous to an adversary as to their supposed master. If you see one I suggest you run, hide or blast it quickly from existence!
Info on the the freaking Quasit:
A quasit’s natural shape is that of a tiny horned, winged, and tailed humanoid, although they are capable of shape-shifting at will. They are normally found serving as counselors, spies, or spellcasters for more powerful demons or chaotic evil spellcasters.
Quasit are poor fighters, but they have many clever tricks up their sleeves, such as poisonous claws and the ability to shapeshift, turn invisible, and cause fear. They generally employ these tricks to the full as they prefer to use guerilla tactics when possible.
SEAN ÄABERG has a trippy Quasit pic he did shared on his blog in this post:
Whazzit? I dunno, but they call it a Quasit.