I feel fairly comfortable answering your second question - It was not the first "horror clown," depending on your definition of "horror clown."

Stephen King's It was published in 1986, and was playing with some of the same spooky circus tropes as Ray Bradbury's Something Wicked This Way Comes (1962 - made into a film in 1983, and directly referenced in a few Stephen King works.

It's been a while since I read the Bradbury, and I don't recall a specific clown character - the real antagonist is the ringmaster - but the story runs on the same fuel: a circus is a chaotic and scary place. There are dozens of carnival horror stories - 1962 also brought Carnival of Souls, but going further back in cinema, there's Todd Browning's Freaks (1932) and The Cabinet of Dr. Caligari (1920). Both Carnival of Souls and Caligari have creepy monster-types in pretty similar white makeup with dark circles under their eyes, though neither are technically clowns.

Stepping back further, though, the opera Pagliacci features a white-faced clown-murderer and came out in the 1890s. The characters within the opera are players in a traditional commedia dell'arte troupe, which is an art form that goes back centuries.

If you're familiar with Punch & Judy shows, they descend from the same theatrical source - there are very similar stock characters and situations, only what Punch & Judy do with puppets, the commedia (and the British "panto" or pantomime) does with makeup and costume.

So - I think your real answer lies in the commedia and harlequinade shows, but most of what I know about them came through osmosis and general reading in sideshow culture, so I can't help you with sources there. But there are characters with white faces doing unpredictable and occasionally terrifying things throughout that tradition... including one usually called "the Clown."