A parasite eats a fish's tongue, then attaches itself and becomes the tongue

Cymothoa exigua, a parasitic isopod, enters through a fish's gills and clamps onto the base of its tongue, cutting off the blood supply until the tongue withers away entirely. The parasite then anchors itself in the tongue's place and lives there, functioning as a replacement organ while the fish continues feeding around it, apparently otherwise unharmed. It's the only known case of a parasite fully and functionally replacing an organ of its host.

— Wikipedia contributors, Cymothoa exigua — Standard zoological accounts of Cymothoidae