I do not know what state you and your mare live, but one of my horses manages to lay down close to fire ants. We can not always see their mounds and if she is laying on top of hay that they have scattered, then the ants could be under the hay. I find tiny bumps along her shoulders and her rump. I also get bit by these fire ants and know from experience that when they bite this area will become sort of infected and a white pus will develop. We just pop this to get rid of it.