Yes, BIGHORNS is a Scrabble word!

Letter values in Scrabble US: B 3, I 1, G 2, H 4, O 1, R 1, N 1, S 1.

Is it a valid word?

BIGHORNS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a river that flows from central Wyoming to the Yellowstone River in southern Montana
wild sheep of mountainous regions of western North America having massive curled horns

Also: bighorn river, bighorn sheep, rocky mountain bighorn, rocky mountain sheep, ovis canadensis.

Not playable in Scrabble US: CIMARRON.