Or, Mythical Creatures you never, ever wanted to know existed
(--Assuming of course you weren't unlucky enough to have found them already.)
I was originally just going to post the link to this impressive resource from the British Library [needs Shockwave plugin], but then I started poking around in it past the reason I originally googled and found it (original Jane Austen MS facsimile), and the Luttrell Psalter sounded sort of familiar, and it was - only not most of it, which I hadn't seen even in my books on How To Do White Vine Lettering &c. The bits that get reproduced in books on Medieval Calligraphy or Arms & Armour are rather dramatically different from the majority of the margin illuminations, most of which are seriously surreal, as you can see in the (alas incomplete) excerpt in the British Library site.
You will hear that Hieronymus Bosch was a totally-unique, utterly-innovative artist whose surreal creations had no precedent in European painting.
Nuh-uh. His style is different - new kind of media, new formats possible in it, it all looks as different as a tune written for tinwhistle sounds when set to full orchestration when it's done in egg-tempera. But the world in which people wearing funnels on their heads engage in bizarre or mundane activities surrounded by people with fish for faces and the legs of weevils on the one hand, and by characters from religious legendry on the other, is the same one where we find people like this guy or this critter or this one. These aren't your run-of-the-mill medieval grotesques, either - not just dragons/griffins or anthropomorphic animals or green men or nudes (although Luttrell Psalter's got those too) - these are Things, kludged together like nothing so much as the fantastic dinner centerpieces of "cockentrice" and other surreal "subtleties" from the feasts of the High Gothic era.
--What does a whatsit like this have to do with, on the one hand, the Crucifixion, and on the other, people engaged in the process of turning grass seeds into dinner, whacking unwanted suitors with distaffs, lying in coffins - or riding off to fight the Blue Meanies in tournament - let alone the Psalms? What does a woman barfing up a bird-rabbit have to do with anything, you might ask?
--One possible answer is that whatever obsolete substance they were ingesting in centuries prior to the exciting new high imported from afar was a good deal more potent and pyschotropic than that "Indian weed" with its oxygen-depriving properties. Otherwise, your guess is as good as mine. (And did the Luttrell family, upon receipt of this opus, go "Whatte the swyve? --How much did we pay for this?!" or did they just figure that it was Art and therefore Meaningful, rather than speculate that the guys at the scriptorium had been experimenting with Fly Agaric before sitting down to the silverpoint...)