Why does Eragon's poor family have horses? This should probably be removed.
As for the book according to the IC wiki:
“Getting answers out of you is harder than dragging Birka through the mud.”— Roran to Eragon, Brisingr, chapter Around the Campfire
Birka was one of the two draft horses employed on Garrow's farm. The other one was Brugh.
Birka means sheep in Hungarian.