"Is the horse at yours?"
i agree -- this is not English. Yes, I know they want me to use עצלך, but "at yours" is not English.
This is a correct British English sentence. If you've missed placed something you might phone a friend and say, "Is the... at yours?". It's an abbreviated sentence but not slang.
On the American side of the pond, we would only say "at yours" if it had already been defined by an alternative. So we might say "It's not at my place, is it at yours?" and that would be perfectly fine, but to just say "Is it at yours?" without the previous phrase to define what of yours (your place) would just not be heard here.
That's the fun part of learning languages. Each of them is a way of thinking about reality