A custom is an item that is significantly altered or scratch-build (not just slightly altered, say where only the bumpers of a model car are painted). Those can be added to hobbyDB with their Production Status marked as Custom. Their Brand should be the Customizer or "Unknown Customizer" if not known.
If they are based on an existing model and still resemble that model they can be varianted to it, like this custom based on the Hot Wheels Kool Kombi:
For customs based on Funko Pops we have a subject called POP! Customs (we had asked the Pop Price Guide community if they wanted these customs to be included on PPG and the answer, for the time being, was no, therefore these do not show on PPG).