I don't know about the dog, attacking a horse is different to attacking a human, she's an XL Bully - Bull - Bull Terrier, it's literally what her breeding is about, taking down large prey animals like bulls, or horses.
It's a logical conclusion to draw that a dog with breeding like that will be more likely to go after a horse if unrestrained and the owner clearly doesn't understand that because he's saying the dog was intimidated by the horse - highly unlikely for a dog of that size, strength and breeding imo.
It's like me taking my JRT somewhere there are small and furries and claiming I didn't know that she'd be likely to do that because she's good with people. I know she's more likely to go after a rabbit or rat because that's what she's bred to do, possibly even a smaller cat. I have trained her with cats, but you can never train instinct out entirely and so she stays on the lead in a situation where there might be a risk. Tbh she grew up with a cat who put her in her place from a tiny puppy, and she's normally wary of cats for that reason, but I wouldn't trust her 100% with one because of her breeding and because she's a relative size - around a horse, not an issue of strength and size, but still capable of causing a problem none the less.
But, these dogs are attacking people too, and I do believe that breeds have traits that we've bred into them and although I'm loathe to condemn a breed because they often attract idiots, unfortunately they attract these idiots for a reason - what we've selectively bred into them. Size, strength and aggression with these types of breed as a show of force and for the 'thrill' of thinking they can control a dog like that.
But I'm clear on the fact that it's the idiots at fault, and I feel desperately sorry for these dogs that end up in these situations because they're doing what instinct tells them, without training to counteract that, and they haven't been given a bloody chance by irresponsible breeders and owners, by even something as simple as putting the dog on a lead in public or keeping them away from children - as you should with any dog.
This owner is beyond belief, playing the victim about people being 'rude' to him and bleating how the dog is his baby girl and keeps him going - I call bollocks, she's a statement dog and that statement is 'Look how hard and Gangsta I am'. And he's right about getting kicked by a horse being dangerous, but it was his responsibility to stop that attack and he didn't even try. Tells me everything I need to know, more bothered about his own neck than stopping her attacking the horse, or trying to protect her from the hooves - if he loved the dog as much as he claims and thought she reacted in fear, why didn't he try and stop it for her protection, even if he didn't give a shit about the horse. I hope that if it gets to court and he has his 'say' it's explained to him in small, on syllable words how fucking ridiculous he is.
That dog is as much of a victim as the horse.
And I say that as a horse lover, that was awful to watch, and it's so lucky the horse didn't go down because he looked like he was going to a couple of times and that would have been catastrophic for the horse and dangerous for the rider and those around.
Been in the horse world a long time and dog attacks aren't rare, I've experienced two and most horsey friends have also had at least one incident - but they're rarely dealt with, and they're becoming more common, along with dog on dog attacks and dog on human attacks. I love my dogs and dogs in general, but, we have a serious problem developing with the growing number of dogs that aren't under control, trained or restrained and people owning them who don't care and are putting their dogs in these situations, as well as the other animals and people getting hurt or worse. Something needs to be done.