Ursus is the Great Cave Bear spirit, the most powerful and revered spirit within the Clan. He is believed to be the most powerful of all the totem spirits and is the totem of all the Clan. In special circumstances, Ursus is believed to become a personal protector for a person; when Creb was a child, he survived an attack by a cave bear which left him permanently scarred, which people took as a sign he had been personally claimed by Ursus. The Clan believe that Ursus is the one who showed their people how to make fire and wear animal skins as clothes.
The Cave Bear is very important to the Clan; they believe cave bears to be sacred, holding a special ritual called the Bear Ceremony at Clan Gatherings in honour of Ursus. Mog-urs also use a cave bear's skull and bones as part of certain rituals, including cursing a person with death. When Brun's clan were lead to the cave found by Ayla after the earthquake, they found a cave bear skull in the back of the cave, leading them to believe they had been intentionally lead to this place by Ursus. A common saying within the Clan is "Walk with Ursus", with Brun signing this to Ayla when she cursed with death. The clan's full name, the Clan of the Cave Bear, specifically refers to Ursus.
Ayla herself invokes the spirit of the Cave Bear when burying Thonolan, asking him to guide Thonolan's spirit to afterlife as she did not know what his totem was. Although Ayla usually calls upon her Cave Lion totem for guidance and protection, she believes she may also be protected by Ursus. As a result, one of the titles she is sometimes referred to amongst her own kind is "Protected by the Cave Bear".
- Ursus is actually the Latin word for "bear".