Ryan Seacrest X-Mas Party -- Dancing Man Arrested ... IN ASSLESS CHAPS

Ryan Seacrest's company X-mas party got REAL cheeky this week in Hollywood -- in fact, some random weirdo was arrested outside on the street ... after dancing all over the place in assless chaps ... and the hilarious scene was all caught on tape.

We have no idea who the mystery man is or why he was there -- but Tuesday night outside Loteria restaurant, this god among men stripped down to assless chaps ... and danced away like there was no tomorrow.

The best part ... he was dancing to Rick Astley's "Never Gonna Give You Up." And he's got some impressive moves too.