Slithy

Slithy, when applied to a monster, is explained in the code as meaning having a "serpent body", but is also used about unserpentine monsters such as jellyfish and sharks.

Slithy monsters may not kick, nor may they jump through mundane means (the spell works) or ride a steed

Having no feet, they cannot wear boots. A slithy monster which is medium-sized or smaller may move through iron bars.