lacchus was an obscure god sometimes called the son of Demeter (or of Persephone) and often identified with Dionysus, although occasionally said to be his son. Dionysus' other name, of course, was Bacchus, and it is possible that Iacchus came to be identified with him because of a resemblance between the two names.

Or again, as with Bacchus (and also Hymen) his name may have evolved from a ritual cry: he was always associated, as Dionysus was, with shouting and revelry. Furthermore, the verb 'to shout' is iacho, so he really seems to be a sort of personalised shout of joy ('Yahoo', so to speak).