I mean, a mule is smarter than a horse. You might talk them into carrying you on their back, but it will be at their speed. And the mule doesn't need anyone to tell him to stop. He stops when he's damn good and ready. And starts when he's damn good and ready.
Greyhounds are pretty low on the cerebral scale, too.
Or maybe it's their eyesight that is bad. I mean, not too many dogs would confuse a piece of fur tied to a mechanical arm for a rabbit. Bugs Bunny would not have anything kind to say about greyhounds. But like the people at a horse track, the people at the dog track will give greyhounds human characteristics. You will hear the latter a lot at the dog track, especially from those people who have lost money on the particular dog in question.