1. File a police report every time something happens. Your police will probably get P.O.'d about it but they will remember, if something does happen.
2. Take pictures once the dogs are on your property. I HOPE there is a leash law in your county.
2.1 Hard as this will be, get pics and/or videos of the dogs deviling your foal, THEN do something about it. How we did something about it was to get on the 4-wheeler with the rifle and shoot over the dog's head.
That Segways into my own story:
1. New neighbor on the road up on the ridge that we can't see his house but his two little Weasel Dogs liked to come into the pasture and devil my horses. I was amazed my horse that normally chases dogs didn't go after them. These dogs were either JR/mixes or Chihuahua/mixes and pretty small so maybe that's why Rusty didn't do anything when one of them got within 12 inches of his nose and started to growl.
2. The neighbor thru the woods on my road is a county deputy; I called him. He knows the guy and said the guy's "always been a nut case and I've dealt with him in the past".
2.1 Deputy goes to the guy's new house, guy gives deputy as much crap as he can without getting arrested, deputy tells man we WILL shoot his dog(s) if they chase our livestock.
3. Next day the Weasel dogs are back at chasing my horses. Mr. WTW lost it (he's Type A so it doesn't take much), he got the rifle, got on the 4-wheeler and went after the dogs who were still deviling my horses.
3.1 He got pretty close to the dogs as they were trying to run under the fence and head home. He shot a couple rounds over their heads that we know the owner heard and probably watched.
4. The dogs haven't been back since and that was two months ago.
5. Another huge help was the Sunday Mr. WTW was chainsawing a tree lightening had destroyed and it was laying on the fence. Our Cur/Fox Terrier and the Dobe/Rott were supervising the chainsawing event.
6. Losers dogs, including his big Golden came down to the fence, barking for all they were worth but did not try to cross the fence<-- I think that was because he has those dogs trained to a silent whistle.
Our dogs have no formal guard training but they are very well mannered. They both stood up, looked at the neighbor's dogs and said "rrrrrrrooooofff".
The dogs wagged their tails and went back up the hill. I know darn well Mr. Loser saw all that happen and now has a whole new respect for the old people living below him that he thought he could bully and do as he pleased.
Meaning, after you file your police report and take pictures, get your gun, even if it's an air pistol and let loose on the dog(s).
Rule Number One in the game of life on the bad side of the sidewalk, is to always let the opposition think you are at least one notch crazier than they are
I hope you can get this resolved quickly. If you work, you may have to shut the mare and foal in until the foal grows big enough that it is no longer viewed as fair game by the dogs.
Or just shoot the dogs; once you file police reports and your concerns are on record, you should be within your rights to kill the dogs if nothing else works.
Our neighbor was warned by the police and we would have killed those two little one in a heartbeat, if need be. Once the owner saw we meant business, he became very diligent at keeping them on his side of the boundary fence.
EDITED TO ADD: JMike posted while I was typing and pretty much said the same thing:)