Yes, WARHORSE is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, R 1, H 4, O 1, R 1, S 1, E 1.

Is it a valid word?

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

a work of art (composition or drama) that is part of the standard repertory but has become hackneyed from much repetition
an experienced person who has been through many battles; someone who has given long service
horse used in war

Also: old-timer, old hand, old stager.

Not playable in Scrabble US: OLDTIMER.