It's the white fur. Their fur usually changes to white in the winter in order to not get eaten. In the spring it's fur changes to a blue/gray color in order to blend In more.
It is the white fur the characteristic of a Arctic rabbit that is specifically an adaptation to living in the tundra. The Tundra is a biome that can be found around the arctic circle. The white fur helps the rabbit to protect it from the cold temperature.