Who were the kitties?

[Renille]

In FOtR, in the chapter "A Journey in the Dark." Aragorn refers to the "cats of Queen Beruthiel."

He will not go astray-if there is any path to find. He has led us in here against all our fears, but he will lead us out again, at whatever cost to himself. He is surer of finding his way home on a dark night than the cats of Queen Beruthiel.

Does anyone have the slightest idea who this queen was? Is the story in the Unfinished Tales? (I'm not finished reading that yet...so I wouldn't know.)