I'm trying to remember a series of stories (I think there was 3-4 books in the series) about a planet hopping adventurer / lawman / aristocrat who had a mechanical horse called Fess or Fez (or something like that). The only thing I can remember is it suffered from seizures at the most inopportune moments.
I wonder if this is the Warlock of Gramarye series by Christopher Stasheff.
In the book The Warlock in Spite of Himself the hero Rod Gallowglass, an agent of an organisation called SCENT, is assisted by an AI called FCC 651919 that he calls Fess for short. Fess controls a mechanical horse and Rod rides this into various adventures, so it is sort of a lawman riding a mechanical horse called Fess.