Is ONE PIECE episode 377 filler?
Canon No, episode 377 of ONE PIECE is canon.
#377
The Pain of My Crewmates Is My Pain! Zoro's Desperate Fight!
仲間の痛みは我が痛みゾロ決死の戦い
Aired 16 Nov 2008
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.