What does John Grady mean when he says, "Horse has nothing to do with it?"

Yes, the horse has nothing to do with it; he's referring to a dialog in which evolution is mentioned. What a strange question............ this is a popular phrase used in conjunction with Darwinism. Very long explanation......... too long for this forum!

When John Grady said "the horses had nothing to do with it" he meant that it was not the horses that made them go to jail but it was his relationship with Alejandra.