I am not a native English speaker, but the sentence: "Is the horse at yours?" sounds very strange to me, while the sentence "Is the horse with you" just sounds normal.
I am a native English speaker and I agree, the only way I could see "Is the horse at yours?" happening naturally is in a really specific conversational context.
Doesn't "is the horse at your house" mean the same thing as "is the horse at your place"?
"The horse is by you" is an expression Jews use in English, but it is by no means correct (in other words, if I had known enough to use it in grammar school, my teacher would have scolded me.). Unless, of course, it was standing beside me.
OK, I guess I should modify my statement: Some Jews use it. Mostly New York Ashkenazim, from my experience. I agree with you, it does sound like bad English. I never used it, myself. I didn't grow up in New York.
I saw a comment on a similar sentence from someone who said the syntax was familiar to them, more like German. Perhaps it would make sense to some Ashkenazim too?
(Didn't see this but again , NY Ashkenazi, I'm also surrounded by Yiddish speakers, maybe you heard it from a specific dialect? I've never heard it, I'll ask the family if they've ever heard it in Yiddish in Litvak or Galitzianer but I'd be really surprised, as I've heard most of the Yiddish expressions they use).
Sorry in the app, can't edit: Ok just conferred. Was told never, but somewhat similar expression is used: A mansion next to yours ... To denote future success.
I'm Jewish as well and it's also not an expression I've ever heard...
הסוס אצלך should be considered right also!