Why do so many medieval manuscripts depict butt trumpets?

I have an earlier answer on...uh...weird-ass medieval art, if you're interested!

Ooh, weird medieval art, everybody's favorite.

The first thing to keep in mind is that basically all medieval art that you're familiar with is by highly skilled artists. Less skilled drawings and illuminations absolutely exist. But they were in manuscripts less likely to be preserved, and far less likely to be promoted by modern libraries digitizing their collections.

When it comes to animals that look...not like their natural equivalents, the most important thing to keep in mind is iconography. This is basically what an image stands for or means, and usually, it's used with respect to a particular image used over and over to represent something. A modern example would be the iconography of "play" for a song or video.

Medieval art is very iconographic, both in terms of themes and in terms of specific images within the theme. So you can represent the entrance to hell as a "mouth", but via Hellmouth 1, Hellmouth 2, Hellmouth 3 &c. Or it can be uniform--the apostle Paul always looks kind of like an Ood.

In a lot of cases, the iconographic value of something, for recognition purposes, was more important in medieval art than realism. Medieval people definitely knew what stab wounds/cuts look like. But in late medieval art, the iconography for the side wound of Christ (where the spear of Longinus pierced Jesus' side while he was hanging on the cross) looking like...let's just say "something else entirely".

So somewhere along the way, various animals got stylized. And later artists copied the tradition, regardless of whether or not they knew the animals firsthand. Obviously, in the case of unicorns, griffins, and other fantastic beasts that were treated right alongside real animals in bestiaries and encyclopedia, no one knew the animals firsthand--yet they still had well-developed iconography.

In some cases, iconography or illustration incorporates both the object/scene being drawn and its allegorical meaning or an interpretation. The illuminations in the original manuscripts of two of 12th century renaissance woman Hildegard of Bingen's great visionary treatises, Scivias and Liber divinorum operum, depict how she describes her visions, but with additional elements that are the interpretations she (God speaking through her) provides of what she saw.

As for the truly weird-ass art? Well...in a lot of cases, scholars aren't sure. Just considering snails in 13th and 14th century art alone: Yves et Françoise Cranga, "L'escargot dans le midi de la France: Approche iconographique" and Lillian Randall, "The Snail in Gothic Marginal Warfare" offer twelve different possible explanations between them.