Lauralie2 inspired me to go searching through my dictionaries:
The full Oxford English Dictionary (2nd edn) 1989, says that the word is occasionally used in the singular. It gives nine quotations from 1580-1886 with the plural form, and six from 1607-1908 with the singular.
Webster’s Third (1961), also mentions the singular form, but gives no quotations – but then it gives no quotations for the plural form with that meaning. On the illustration under horse, it gives only the plural form
The Oxford Advanced Learner’s (6th edn), 2000, is the only one to mention AmE
My other dictionaries give only the plural form and make no mention of BrE/AmE. They are:
Sam Johnson’s, 1755; Odham’s, 1946; Virtue’s Simplified, 1943, The Concise Oxford (9th edn), 1995.
I have certainly heard it used in Britain, and nobody has come up with a BrE version yet, I wonder while ALD thinks it is AmE.
I was surprise to read that BobK has usually seen it in the singular, but then I have little to do with horses, soI haven't encountered it often.
One more thing, BobK you write: "No, it's Br Eng too. And - like scissors and trousers - I've usually seen it used in the singular."
Do you really mean that you usually see these two underlined nouns in the singular form?