Definition of "pony"

‘I’m on the inside track,’ said a pony of beer as it went galloping down a man’s throat.

1885, New York Journal, August

Demon popped into his mouth a last morsel of black bread with elastic samlet, gulped down a last pony of vodka and took his place at the table with Marina facing him across its oblong length.

1969, Vladimir Nabokov, Ada or Ardor, Penguin, published 2011, page 193

I did not even know what a ‘pony’, a small chaser of beer, was. But of course I could not admit that. So putting on an air of nonchalance, and a deep voice, I strolled into a pub with one of the other equally naive guys and we ordered two ponies of beer.‘McEwans?’ asked the barman.‘Naw - ponies’ said I.

2010, Dick Lynas, Pies Were for Thursdays: Tales from an Ordinary Glasgow East End Childhood, page 283

“You said a pony,” interposed Clavering; “my dear fellow, you said a pony, and I’ll be eternally obliged to you; and I’ll not take it as a gift—only as a loan, and pay you back in six months. I take my oath, I will.”“Well—well—there’s the money, Sir Francis Clavering. […] Here’s five-and-twenty for you.

1848 November – 1850 December, William Makepeace Thackeray, chapter 61, in The History of Pendennis. […], volumes (please specify |volume=I or II), London: Bradbury and Evans, […], published 1849–1850

She kept the dates written down in her Latin 'pony', so she didn't have to bother about who it was.

1931, William Faulkner, Sanctuary, Library of America, published 1985, page 104

His hair is a semilong dull red and pulled back in a sloppy pony at the base of his neck, and his face is riddled with small freckles and grease, but out here I can see personal hygiene isn't exactly at the top of the priority list.

2012, Amlynn Smith, Lost and Found, page 18

He suggested a Rose Ballet in which he would use the G. V. Follies chorus girls, chiefly the ponies and mediums.

1941, Thoda Cocroft, Great Names and how They are Made, page 140