Green’s Dictionary of Slang

(orig. US) a mustang (esp. a mare) with a short bushy tail.

1922		(con. 1890s) P.A. Rollins Cowboy 28: Both South and North might betake themselves to slang and talk of ‘fuzzies’ (Range horses) and ‘broomies’ or ‘broom tails’ (Range mares).
1945		in DARE.
1967		J. Schaefer Mavericks (1968) 140: He was usin that thing to spot lil ol’ broomies back in the canyons an’ chouse ’em down onto the flats. [Ibid.] 156: Come – on – you – lil ol’- broomies. Head-for-the-hills!