Hoki the Jokester, a Ramtops god of practical jokes. He most commonly manifests himself in the aspect of a flute-playing half-goat/half-man (a la Pan), though Granny Weatherwax considers him a bloody nuisance (Equal Rites). He was banished from Dunmanifestin after pulling the old exploding mistletoe joke on Blind Io, chief of the gods (see The Discworld Companion). Hoki has not yet featured in a Discworld novel, though Rincewind does swear by him ("or my name isn't Rincewand, by Hoki", intentionally mis-stating his name) in The Last Continent.
The reference is to Roundworld deity Loki, the trickster-god of the Nordic pantheon. Among other terminally effective practical jokes, he slew the naive and trusting god Baldur with a dart fashioned from mistletoe, after spotting a loophole in the rules of the Game. Apparently no living thing with its roots in the earth was capable of slaying the beloved god. But Loki realised that mistletoe, a parasite on oak trees, does not have its roots in the earth. So during a jolly game where the rest of the gods amused themselves by seeking to slay Baldur with spears (made from trees which root in the earth) swords (made from iron which is drawn from the earth) and rocks (the very roots of the earth) to no avail - Gods everywhere have a sense of humour akin to that of rugby club members - Loki sidled up and offered to try out his weapon. Laughing and expecting it to bounce off, Baldur bared his breast, and was slain on the spot. The rest of the Gods immediately sobered up, and Loki's penance was to be chained to a rock for all eternity, fending off venom dripping on his face and body. So Hoki got off lightly with mere banishment...
All pantheons appear to have their trickster god, whose function appears to be to teach and drive home lessons through painful and embarrassing practical jokes. The Greeks had Eris/Discordia, who was responsible for the Golden Apple stunt which got the Gods fighting among themselves and was indirectly responsible for the Trojan War. The Celts had Morrigan, a similarly vicious practical joker who manifested as a Raven (even today, the raven is a bird treated with wary respect in rural Ireland), and the Raven as Trickster motif is also to be found in Africa. In West African, West Indies and South American folklore, Anansi, the spider, fills the niche, and South-Western American Indians talk, although not very loudly, about Coyote.
It is possible the Trickster on Discworld is one of those Gods who moonlight under many guises to garner more believers, even if all they believe is "that bloody nuisance is back again". Hence the cynical disbelief on the kangaroo-god's face when Rincewind swears his oath "or my name isn't Rincewand, by Hoki"... he might as well be talking to Hoki himself!
In the Discworld Snuff the riverboat captain Gastric Sillitoe swears to Samuel Vimes, "Nevertheless, I can see you're a man, sir, who does what he sees needs doing, and by Hokey, I can't argue with that!"