I know that there are orcs, but that doesn't mean that there are half-orcs. So, the question is pretty simple: do they exist?

I don't remember there being orcs, but I am fallible. If there are orcs, half-orcs are possible. As Ravnica is a known multiversal point even if orcs are not natively originating there they can find a way there.

You're right, I might have been looking at a list of races that was wrong, or I was thinking of ogres, instead. 

Thanks for the answer, though.