They've been best friends at least since they were 6 years old - likely longer.

Despite Horace being shown to be a major jackass to the point of committing murder when he's first introduced, in a flashback sequence we see him with Simon, he speaks notably more kindly, and seems to significantly soften up when it comes to him.

Simon blames Horace for the death of his father, and winds up getting him killed for it. However, he seems to be incredibly reluctant to kill him and even basically says he wishes it hadn't had to turn out like this, while he doesn't show even a little bit of remorse for any of the other people he killed.

Canonically, Simon is representative of the king piece in chess, while Horace is representative of the knight piece.