*hratan
On second thought, it is also unclear where the verb rit comes from. The Middle English Dictionary says that it derives from unattested Old English *rittan not *hrītan. rit is pronounced with a short i, by the way, and rat also has a short vowel.
Another thing that keeps bugging me is that the verb rat has a form to-rat which is very similar to Dutch terijten and German zerreißen and as far as I know the verb rit does not have such a form. Why would it have such a form (i.e., to-rat) if it did not come from *hrītaną? I mean, there is a rather small number of words out there that are prefixed with to- in English (including dialects and late Middle English) and almost all such words have cognates in German, Dutch and other Germanic languages. How would you explain that?