Sometimes prices are firm, other times the seller is negotiable. If it were me I wouldn't just out & out make a lower offer, I would ask the seller if the price is at all negotiable. If she says no, then you know there is no point offering less; if the answer is yes, then you can either make an offer, or ask for more specifics on how much less she would be willing to accept. For myself, I consider if the horse is worth the asking price--is that horse worth that particular price to me? If I am willing to pay $500 less than the asking price, how likely is it that the seller will find a buyer at full price...if the horse is just perfect & the price is right, but you want to wait until the seller will take $500 less, if someone else comes along tomorrow & pays full price, will you be kicking yourself for not just buying, rather than waiting to have an offer accepted when the horse doesn't sell? Or will you be happy to walk away saying oh well, the horse really wasn't worth that much money & I'll just buy something else? All things to consider when dickering on a horse!
There's no rule that says the set price is always high and the buyer is expected to offer less.