Ursa Major

An Ursa Major is a large purple bear and a parent to the Ursa Minor. It was mentioned several times in the episode Boast Busters when Trixie claimed to have defeated one, but she confessed that she made it all up when an Ursa Minor attacked Ponyville.

The Ursa Major didn't make an appearance until near the end of the episode in its den. When Spike asked what an Ursa Major looks like, Twilight Sparkle imagines the Ursa Minor being cradled by its parent, the Ursa Major, after it was placed back asleep in its den by Twilight's magic. She replies to him 'You don't want to know.'

Trivia

 * The names Ursa Minor and Ursa Major are both star constellations in the northern sky, also known as the Little Bear and Big Bear, or Little Dipper and Big Dipper.