That’s not the way it works. The seller can fudge the paperwork for the crossing to lower the tariff, which comes with its own issues (insurance for the full purchase price is one that comes to mind), but you’re not paying the tariff to the seller, you’re paying it to the US/CBP. Horses are at least someone with a very fluid price that your average CBP agent is not going to question, unlike lots of merchandise coming into the US with an established valuation table.
It would give me pause if the sellers are willing to take what is essentially a 25% price reduction if they are “absorbing” the increase. Are they jacking up the price to cover the difference or were they charging US buyers way more than the going rate for the horse?
Unless they are doing a door to door delivery so handling the import/border crossing for you - it is the CBP that collects tariffs. I’d make absolutely sure the sellers provide you with a receipt for the paid tariff so if the CBP reaches out for payment of the tariff you have proof that it was paid. Not saying the barn is unscrupulous, but I’d worry that they have a border crossing agent in their pocket that lets them through with little hassle and then you end up getting a bill in the mail for the tariff
At least for livestock, needing a USDA vet available for the crossing and them being, ya know, live animals, you won’t end up with the horse sitting in a customs warehouse for weeks/months on end.