Yes, HORSETAIL is a Scrabble word!

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

Is it a valid word?

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

perennial rushlike flowerless herbs with jointed hollow stems and narrow toothlike leaves that spread by creeping rhizomes; tend to become weedy; common in northern hemisphere; some in Africa and South America