I have a feeling I’ve read somewhere that Past Participles can be used as nouns or adjectives with their masculine -us ending forms being treated as nouns of the Fourth Declension.
Am I misremembering?
Some for sure.
pōtus from pōtō
volātus from volō
Wiktionary has some information on this: -tus - Wiktionary, the free dictionary
These are different PIE suffixes that happened to have merged in Latin.