OP I realize that this must be horrible for you and that you feel dreadfully responsible. Tear the Electrobraid out if it suits you to do so.
But two times my buddy has come home, once to a rising two year old colt stone dead in the field, kicked dead center in the forehead, and the second time to that same colt’s grand dam unable to rise with a broken scapula. Euthe’d that afternoon. Both valuable and loved animals and both dead through pasture accidents.
I’ve witnessed at my trainer’s, four board fence, a filly kick high at her “enemy” and thank God we were all there and saw her go right through the fence. She was hanging there. Vet was there on another call with sedatives available, Roberto took that fence apart so fast . . . Only scrapes. Could have been just as deadly if the filly had panicked.
They survive the most unsurvivable looking accidents in trailers, and they walk five steps and lay down and breathe their last on some fine summer morning days after having gone to some fun and easy event.
There is no rhyme or reason to it, you gave that horse the best care you could. My deepest sympathies to the owner and yourself.