Is there any source of the creation of an Elder Brain?

I've not found anything clear on the subject, but I have a theory that the Elder Brain is an unrelated psionic creature that has a somewhat symbiotic relationship with Illithid communities (like how some other creatures live near Mindflayers to feed off of psionic energy in the area). Though I could believe it's the "final form" of a Neothelid that reached critical mass, and has to shed the body.