Yes, HORSEMINT is a Scrabble word!

Letter values in Scrabble US: H 4, O 1, R 1, S 1, E 1, M 3, I 1, N 1, T 1.

Is it a valid word?

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

an annual horsemint of central and western United States and northern Mexico
tall erect perennial or annual having lanceolate leaves and heads of purple-spotted creamy flowers; many subspecies grown from eastern to southwestern United States and in Mexico
a coarse Old World wild water mint having long leaves and spikelike clusters of flowers; naturalized in the eastern United States

Also: lemon mint, monarda citriodora, monarda punctata, mentha longifolia.