Since I wrote about Mom's inexplicable experience a few days ago, I've discussed it further with Kim, who helped to clarify a few things. First off, my timeline was slightly off; Kim said that the year was 1966, not 1964 or 1965; she was 5 at the time it occurred, and remembers some details of it from later retellings.
The horse that lived in the "pasture" behind us was named Superman (appropriate for a horse located near a comics fan, huh?). He was a huge horse with a broad back; what was so odd about that--and the source of such amusement for Kim and her friend--was that there were a couple of cats who would actually sleep on Superman's back at times; they'd climb up on a ledge, jump lightly on to that broad back, then settle down for a nap. Superman didn't seem to care; he tolerated his feline companions on multiple occasions.
The night that the event occurred, Mom was sure that she saw the strange figures doing something to Superman.
Superman died the day after this occurred. His owners came and took the body away as soon as they learned of his death, so none of us ever knew what caused it or what condition he was in. After Superman died, the owner of the property didn't put another horse in there.
The pasture went into further decline, the barn collapsed eventually, and the land was abandoned for more than two decades before someone eventually bought it, levelled everything, and built apartments there.