Sadly there are scammers on Howrse too. Most of the community is friendly but there are some scammers. To help you either get your horse back or try not to get scammed here are some helpful tips.
How to avoid getting scammedEdit
Most scammers will look through sales and if you have a good horse they want they'll offer something (bmi, horses, money, passes, etc.) but they'll ask you to go first. To try to avoid getting scammed if someone offers something through message and it seems unrealistic get them to either prove that they have the items or just say that you don't want to go first. If you think somebody may be a scammer, go to their page, look at their horses then go look at one, then click on history of owners and then you can see how much they bought it for and go through a couple like this and if they all seem quite unrealistic it's most likely a scammer! Edit
How do I get my horse back?Edit
You could try to contact the scammer yourself, but most likely he won't give your horse back. It's useless to go to a moderator, because trading a horse for Black Market items isn't an allowed trade.