It's "Poro"

"Poro" is a cute creature designed by Riot Company. It is a fluffy little sheep-like creature that lives in the Howling Abyss. This The little ones don't do any damage, and you can find two of them in the grass in the center of the map.

Braum comes from the Freljord. If you have seen the promotional animation of Braum's hero, you will find that Braum saved Poro.