The reason is that hapax means "one time" in Greek, while legomenon means "that which is said". So, the noun is legomenon, which will be the word in plural. A

The reason is that hapax means "one time" in Greek, while legomenon means "that which is said". So, the noun is legomenon, which will be the word in plural. A (not very good) analogy is "visible phenomenon". You would not say "visibles phenomenon", but "visible phenomena".