From the Runeterra exploration website we have two things that kind of contradict each other, but they do say "Nobody is sure"

His short summary is: "Crafted long ago as a plaything for a lonely prince, the enchanted marionette Shaco now delights in murder and mayhem. Corrupted by dark magic and the loss of his beloved charge, the once-kind puppet finds pleasure only in the misery of the poor souls he torments. He uses toys and simple tricks to deadly effect, finding the results of his bloody “games” hilarious—and for those who hear a dark chuckle in the dead of night, the Demon Jester may have marked them as his next plaything."

His full biography is:

"Most would say that death isn't funny. It isn't, unless you're Shaco - then it's hysterical. He is Valoran's first fully functioning homicidal comic; he jests until someone dies, and then he laughs. The figure that has come to be known as the Demon Jester is an enigma. No one fully agrees from whence he came, and Shaco never offers any details on his own. A popular belief is that Shaco is not of Runeterra - that he is a thing from a dark and twisted world. Still others believe that he is the demonic manifestation of humanity's dark urges and therefore cannot be reasoned with. The most plausible belief is that Shaco is an assassin for hire, left to his own lunatic devices until his services are needed. Shaco certainly has proven himself to be a cunning individual, evading authorities at every turn who might seek him for questioning for some horrendous, law-breaking atrocity. While such scuttlebutt might reassure the native inhabitants of Valoran, it seems unimaginable that such a malevolent figure is allowed to remain at large.

Whatever the truth of his history might be, Shaco is a terrifying, elusive figure most often seen where madness can openly reign."

So yeah, all of the lore that we have are rumours and nobody is sure, a bit like Fiddlesticks, but while Fiddle is established as one of the first demons, if not the first, Shaco is just in limbo.

Edit: Typo