lol Beaver got nothing but rodents.

That リシリムクゲネズミ there, according to what I can read on Japanese Wikipedia is a subspecies of Hokkaido red-backed vole (Latin name Clethrionomys rex rex). It has 'Rishiri' on it's name, so maybe it's named from Rishiri island?
English wikipedia didn't cover that up.

Snow Beaver wants a common beaver.
Gets nothing but mythic rare rodents.

Mountain Fortress wants a common Fusou.
Gets nothing but common mountain fortresses.

According to here, it's called Rishiri dark red-backed vole in English.