Animated objects don't display correctly in display cases. it's not the only one, and a fix may be some time coming. See also spirited mandrake and a few other objects.
edit--a link (with dev reply):
viewtopic.php?f=47&t=61738&p=783673&hilit=display+case#p7836592nd edit: I'll note that the article that jorb links in his post is referencing OpenGL earlier than 3.x, which the rendering engine has been recently updated to. It may be an easier fix with 3.x, but it still isn't an easy fix.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.