Is ONE PIECE episode 238 filler?
Canon No, episode 238 of ONE PIECE is canon.
#238
Gum-Gum Human vs. Fire-Breathing Cyborg!
ゴムゴム人間ＶＳ火を吹く改造人間
Aired 10 Jul 2005
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.