"A Bayesian is one who, vaguely expecting a horse, and catching a glimpse of a donkey, strongly believes he has seen a mule."In that case, everyone is a Bayesian. Researchers in perceptual science have shown over and over that background knowledge and expectation can strongly influence perception, especially when the visual stimulus is unclear. Anecdotal cases of this abound; e.g., the 1950s TV show in which people say (roughly) "Look, up in the sky, it's a bird, no, it's a plane, no, it's Superman!" Their prior knowledge indicates that flying things should be birds or airplanes, and it's only with accumulating visual data that the prior is overwhelmed.
And there is an error in the joke. If an actual Bayesian vaguely expects a horse and catches only a glimpse of a donkey, then s/he believes s/he has seen a mule only with great uncertainty, not strongly.