- "Polar bear. The largest land predator in the world."
- "Rachel wrapped her own big paws around him and together we wrestled Hulk Frozen to the ice. Tobias swooped down out of the sky, complaining about there being no lift at all in this cold air. Like that was the major drama. He sank talons into Derek's friend Nanook and began to acquire him, while Rachel and I lay panting and counting our wounds. The bear went into the acquiring trance and a few minutes later we all had his DNA floating around inside us."
The Animorphs acquire their polar bear morphs in The Extreme. While struggling in the North Pole as they lacked Arctic morphs of any kind, the Animorphs encountered Nanook, a polar bear, with Marco suggesting they acquire him. Shortly after, they meet Derek, a jocular Inuit who states that Nanook is his friend. Marco asks Derek to take them to Nanook so that they may acquire him. Rachel and Marco morph into their grizzly bear and gorilla morphs and tackle Nanook, beating him into submission. While restraining him, Tobias swoops down and acquires him, placing Nanook in the acquiring trance. Jake and Cassie then acquire him as well, extending the trance while Marco and Rachel demorph and acquire him as well, running away before the trance could fade. In The Ultimate, Liam acquired one of the polar bears at The Gardens, with morph-capable Controllers following suit in The Answer.
- "We had the morph for this place. Like being a tiger in the jungle or a crocodile in a swamp, we owned this place now. Owned it! I've been a gorilla. I've been a rhinoceros. I've felt power before. But this was new. [...] We sauntered back toward the Yeerk base with the cockiness of Clint Eastwood going into the town saloon. It was a long walk, punctuated by refreshing plunges into the icy water. We ended up having to demorph, of course, and that was no fun at all. But then it was back to being Lords of the Ice."
As the largest land predator on Earth, the polar bear was considered to be perfect morph for the Animorphs to use while being stranded in the North Pole. Possessing raw power and sheer strength, the polar bear is the Animorphs' most powerful morph. The Animorphs used this in The Extreme to traverse the Arctic environment and to infiltrate the Yeerks' base. Cassie also partially morphed the polar bear in Elfangor's Secret in order to become "white" after a racist man confronted her for being black.
- "If you guys are really that worried, we'll go in with maximum firepower. We all go in as polar bears!"
"Did I hear you correctly? 'Cause I don't see anything wrong with our usual battle morphs."
"He's got a point, Rachel. We know our morphs. They've been working for us all day. We handle them best."
"We're going for mass here, people. Bulk. Spectacle. Going out in style. Besides, we want to send the message that there are a lot of us."
- ―Rachel, Marco and Cassie
The Animorphs, minus Jake, adopt the polar bear as their new battle morph under Rachel's leadership in The Weakness, since the polar bear is the strongest morph they possess. However, they discover the hard way that the polar bear becomes sluggish at high temperatures and it is not a suitable morph for their hometown environment; additionally, Ax remarks that they have no versatility or flexibility by having the same morph as everyone's battle morph, and thus they all struggle from the same weaknesses. Following this instance, the Animorphs revert back to their original preferred battle morphs.
- The Stranger (mentioned and seen at The Gardens)
- The Extreme (Nanook; acquired and morphed by Jake, Rachel, Tobias, Cassie, Marco and Ax)
- Elfangor's Secret (partially morphed by Cassie)
- The Proposal (mixed with poodle DNA and morphed by Marco)
- The Weakness (morphed by Rachel, Tobias, Cassie, Marco and Ax)
- The Ultimate (acquired and morphed by Liam)
- The Answer (morphed by Liam and morph-capable Controllers)
- The Beginning (morphed by a human-Controller to kill Rachel)