I agree that tundras should be more perilous, even though I wasn't a fan of the Ice Basilisk. I think that these modified animals are a better choice, personally. They should also start with resist cold. I would like to see the cold weather terrain occasionally inflict a small amount of cold damage (frostbite. I think fire and burning sands should do damage the same way, but fire damage, obviously), and for cold damage to occasionally cause a unit to freeze . Perhaps the impassible ice should also expand and recede at random, so both land and sea units could get trapped.
Is there any reason to call it an Ice Bear and not simply a Polar Bear? Perhaps the polar bear should have water-walking (limited to coat, like in the case of The Drown. Polar bears swim to catch most of their food). New polar sea animals would also make since.
I don't suppose there is a chance for any surviving Frostlings to pop up in the Tundras late in the game, is there? These could be weak units that do only cold damage and perhaps could cast water, air, and death spells, unless an ice sphere were to be added.
I would like to see a larger variety of animals within the basic types they have now. They could also add a White (Siberian type) Tiger for tundras who differs from the normal ones the same way your two new animals differ from their normal versions. Perhaps there should be some new non-polar varieties as well, like a Mountain Lion (lion without a mane, a little weaker, but starting with guerrilla I & II).