Zanclidae contains one living genus and one living species. Fossil zanclids are known, so the family was once broader, but every other lineage has gone. The Moorish idol's closest surviving relatives are the acanthurids, the surgeonfish and tangs, which it resembles in body plan but not in its long dorsal filament or its sponge-heavy diet.
The pennant coralfish, Heniochus acuminatus, is a butterflyfish with a similar white body, black bands and elongated dorsal streamer, and the two are constantly confused in photographs. The reliable separations are the snout, which is long and tubular in the Moorish idol, and the yellow area, which sits on the flank and tail of the idol rather than only on the fins.
Moorish idols are notoriously difficult to keep in aquaria because of their specialised sponge and tunicate diet, and most captive imports die. The species itself is widespread and not considered threatened, which is why the trade continues despite the poor survival record.