Yona is the Lapine name for a hedgehog. The plural is yonil.
"A yona. What harm does a yona do to anything but slugs and beetles? And what can eat a yona?" - Blackberry, about the dead yona.
Yona appears in the story, "The King's Lettuce" as a mythical character. Yona's loyalty often wavers because of her penchant for gossip. Before the rabbits travelling to Watership Down cross a road, they notice a yona crushed in the middle.
In the animated series, Yona made her debut in Home On The Down and she is good friends with Hannah, though she appears as a minor character. She helped the other rabbits trick Vervain into thinking it's all magical in The Great Game. On Frith's Eve, she comes to the burrow for shelter from the cold in Winter on Watership Down.
In season three, she nearly rolls on top of Hannah when the mouse accidentally hits her with an acorn. Upon asking Hannah why she is in such a bad mood, she tells the mouse about the Hedge Wizard, and shows Hannah the way to Hedge's island.
She also helped the others to defend against Woundwort and his army by rolling in quills with her hedgehog friends down the hill towards the Darkhaven army. As most of them didn't make it back, their fates are left unknown.