Tempest Rim is a natural formation in Red Dead Redemption 2, in the Grizzlies West region of the Ambarino territory. It’s also referred to as the "unreachable plateau", due to the player not being able to access it.
The area appears to be a remnant of an earlier version of the Ambarino region, where the northern, snow-covered parts of the region were slightly larger and had more routes to and within it.
Much of this area was redone partway through production to improve the flow of the story -- this includes moving Colter from what became Tempest Rim and blocking off the area from the rest of the map.
Animals are not stuck in place in the snowless parts like they are in the snowy parts.
A Grizzly can spawn in the warmer, snowless part of the area and will attack the player.