Sounds like a scare tactic to see if you'll fork over the money without them actually having to sue.
I work for a personal injury attorney and there's NO WAY we'd ever take that case. IMO, they don't have a leg to stand on unless they were actually struck by the vehicle or the machinery. Even then... the road is for VEHICLES and people riding horses do not have the right of way.
I would call their bluff, and do nothing until you actually have something official. Honestly I'm wondering whether anything at all even happened, or whether they're just scam artists.
If you do get something official, get an attorney and countersue as has been recommended.