pony up

Reader's Digest 9月号は8th Annual Humour Issueと言うことで、ジョークや面白い話の特集がありましたが、これはジョークではなく、実話の様で、その中に覚えたい表現がありました。
An Australian man was ordered to pony up $500 in court costs after he lost his lawsuit against his ex-wife. He'd taken her to court hoping the judge would order her to remove this bumper sticker from her car: "Men are idiots. I divorced the king!"
ここに出てくる "pony up" は文脈からすると「支払う」の意味だと思いますが、何故この様な意味があるのか不思議です。まずは意味を確認します。
・Merriam-Webster's Online Dictionary: to pay (money) especially in settlement of an account
・Dictionary.com: Informal . to pay (money), as in settling an account: Next week you'll have to pony up the balance of the loan.
・American Heritage Dictionary: Pay money that is owed or due, as in Come on, it's time you ponied up this month's rent . The allusion in this expression is unclear.
語源はOnline Etymologyに次ぎの説明がありました。
1824, in pony up "to pay," said to be from slang use of L. legem pone to mean "money" (first recorded 16c.), because this was the title of the Psalm for March 25, a Quarter Day and the first payday of the year (the Psalm's first line is Legem pone michi domine viam iustificacionum "Teach me, O Lord,
これでは覚えるための参考にはならないですね。