Here are some possible explanations:
* "Buy Back" in Financial Terms: The phrase "buy back" is often used in finance to refer to a company repurchasing its own stock. This is unrelated to horses or ponies.
* "Pony" as a Slang Term: "Pony" can be slang for a small amount of money. It's possible the phrase is referencing someone having to pay a small amount of money to get something back. However, this is very unlikely in the context of horses.
* A Misunderstanding: It's very possible that the phrase was misheard or misinterpreted. There might be a similar phrase with a clear meaning that was mistaken for "buy back pony."
To understand what the phrase is meant to convey, you'd need more context. Perhaps you could provide the situation where you heard or read this phrase.
If you could give me more details, I might be able to give you a more specific and accurate answer.