It is nothing serious (at least we're pretty sure). She didn't need surgery. Turns out the tests indicated the stuff in the cyst was blood and lymph material - so either an infection, or a hemotoma-like thing, probably caused by her falling onto something (she was born with cerebellar hypoplasia). Can't think (or find) anything she could have fallen on that would cause a bruise on her neck like that, but either way, the decision was to put her on antibiotics for 10 days and keep an eye on the lump.
Thank you all for the vibes - looks like they worked!
Just curious--do you have a video of Ming Loy? How bad is her CH? I've never seen one come in our shelter, and I want to be prepared to stick up for it if it ever happens.