Disclaimer: If it's not already obvious, I understand I am not a trainer, nor is this a riding problem. It's a ground problem that needs to be broken, but I'm not even sure how to go about fixing.
Training Level: Just ground work
We got her as an untouched yearling. First time we saw her, she was shivering, drenched in sweat, and shaking in the pen because her owner stupidly penned her with his stallion and he (the horse) ran her until she could barely stand.
She's aggressive with children and small animals. She's totally respectful of adults and an absolute dream to work with. Other horses easily put her in her place. She's bottom of the pecking order. The problem first emerged when we put her with our two ponies (all three yearlings). She would chase them all day long, every day. We pulled them from the pen and she got put with the riding mares when all three started dropping weight and one filly was developing a cough. She double barreled a dog that was chasing the horses, but that was just dismissed as regular horse behavior. Protect the herd right?
During a blizzard we had the horses in the barn with no access to the outside as we temporarily had an extremely late filly we'd snatched from someone we knew for fear that the weather would take her out. A little boy was helping me with chores (3) and he slipped into the stall behind me. This was not supposed to happen. He's not allowed in the stalls. His grandfather was supposed to be watching him and was not at that moment. She came after him (she's 1 and a half at this point), teeth bared and head down. I managed to snatch his coat and swing him out of the stall right away. The minute he was gone her head came up and she walked up to me calm as can be. Behavior was dismissed as stall energy. (I know moving the feet is the worst possible response to a charge, but I'm not going to make a baby stand his ground and my instincts took over. Plus I've never seen a horse with that kind of murder in her eyes)
Fast forward to last summer. We were getting some goats and were 'goat proofing' our fence. I had my dog with me. My dog was near me in the south-western corner of our three acre square pasture. The two horses were in the north-eastern end. The dog was sniffing in the grass no where near the horses when she caught sight of her. The filly charged again, with the same behavior she'd had towards the little boy. Because my dog is a (smart) coward, she ran.
A week later, we got the goats and while transferring from the trailer to the paddock one goat escaped. To make this short and sweet, she again charged from a long ways away, but this time she actually got her teeth into her target. And by that I mean she savaged the goat (who is fine now). She tore at the poor animal with teeth and hooves before getting a good grip on the goats back and throwing her over the fence. Needless to say I am pretty d*mn glad I reacted as fast as I did with the little boy and my dog ran. If she had done the same to either of them...
Conclusion (And Slightly Shorthand):
My thing is, why is she so specifically aggressive and how do you stop it? Mainly dogs, goats, and small children are at risk. She's just fine with a 7 year old, but has charged at a 5 year old (the children were young, but were behaving well at the time). She tolerates the cats but barely.
This is something I'd prefer to fix before she goes to the trainers. (She'll be going to a roper. We often raise foals for him (he's the son of the man who owns the farm) and we've never had this problem before. We believe it's something particular to her.)
And... As thanks for reading this (and because she's beautiful) here's a picture (or four) of our fiery redhead Chase! (With the most fitting name of any horse I've ever met)
Following two are as a two year-old:
Following two are as a yearling:
And this one's not the greatest picture, but I love it: