A hedgehog.
Get the entire Watership Down LitChart as a printable PDF.

Yona Term Timeline in Watership Down
The timeline below shows where the term Yona appears in Watership Down. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 10: The Road and the Common
...fact that the hrududus are not dangerous—but when Hazel spots a flattened, bloody hegdgehog, or yona, he knows that the hrududus must be powerful. Hazel tells the group to cross quickly.
(full context)