what about "fast as the hare". Would "luath ri an gearr" work?
Were there an actual hare for the dog to have been as fast as, I think you'd need "Bha an cù cho luath ri a' gheàrr", as 'geàrr' is a feminine noun.
This is a simile. There is no actual hare. Just like in the phrase "dead as a doornail."