Ursas were a race of bear-like creatures that inhabited caves of the Everfree Forest. There were two kinds of Ursas: Ursa Minors and Ursa Majors. So far, a single Ursa Minor and a single Ursa Major appeared in Boast Busters.
An Ursa Minor was sleeping soundly in its cave until it was awakened by Snips and Snails, who were looking for an Ursa Major for Trixie to vanquish. Enraged, the Ursa Minor chased the two ponies to Ponyville. There, Trixie attempted to tame the beast but failed miserably. It was thanks to the efforts of Twilight Sparkle that the Ursa Minor was put back to sleep and returned safely to his parent, the Ursa Major.
An Ursa Major is the parent of an Ursa Minor seen in Boast Busters. The Ursa Major was seen tending to its young one, the Ursa Minor, after the latter returned home from rampaging in Ponyville. Trixie claims to have vanquished an Ursa Major but this later proved to be a false statement as she was unable to defeat the Ursa Minor. Spike initially thought the Ursa Minor was a Major but Twilight Sparkle corrected him, saying that they didn't want to know what an Ursa Major looked like.