I tend to agree with this. I may not be experienced with horses but I know dogs very well.
I'd lean towards a pack of coyotes. Hamstringing, killing and eating their prey is a pretty standard pattern for them. I highly doubt a domesticated dog is going to find, stalk, hamstring and partially eat a mini horse. The pattern just doesn't fit. I find it hard to accept a solitary dog did this. Minis although small are still larger and heavier than a dog. They can run longer and no doubt would kick to defend itself. Whatever attacked your mini was determined and didn't do it for sport. It was hungry and killed to eat. Coyotes, wolves, or large predator cat is my guess.
A hungry wolf or pack of coyotes is a different story.
Oh and PS:
1) Pumpkin was a stunted shetland. If I had to guess, he weighed...MAYBE 200 pounds. He was VERY SMALL, and still built like a baby, deerlike.
He had deformed rear legs and front legs. Still think he can outrun a stupid dog?
He tried his best...I saw the tracks. I saw where the dog grabbed a leg and was walking sideways while Pumpkin struggled. Want pictures? Because we have them. We showed them to the judge, at the trial.
2) A dog of that breed almost killed my mother and aunt. They were a VERY long time in the hospital receiving skin grafts, and surgeries which didn't all take. Ask her whether or not it was a pack of coyotes that took her breasts, parts of her forearms and her calves.
Ask her how fun it is to wear long sleeved shirts in the desert.
3) My husband corrects me and says there were THREE AC calls on this dog in the two weeks prior. The ones I remember are the one where he jumped on some guy, and the other where he was growling barking and being aggressive. He was chasing my horses. My other three were limping like crazy the day after the attack.
4) I'm still planning to look up wolves, and big cats in my area today, but I'll tell you right now, the only plausible idea you provided was the "pack of coyotes", and again....newborn piglets squealing their heads off x a lot....obnoxious running horses, hmmm.....