Is there any historical precedent for a medieval “Thieves Guid” as depicted in works like Cervantes’ Rinconete y Cortadillo? Was the concept purely fantasy, or was it based on actual documented instances of well-organized groups of thieves modeling themselves in the style of a medieval guild?

I addressed this question, at least in respect of the possibility of formal, organised thieves' guilds, in an earlier answer which can be read here.

The closest thing to the sort of organisation you are actually thinking of may have been the (very loosely organised) Banu Sasan, a group that flourished in the medieval Islamic world. I described them and their methods in an article, originally written for the Smithsonian Magazine, that you can read here while you're waiting for fresh responses to your query.