If the horse was 12 feet away or so, I don't think she was kicking out at it. Maybe in frustration to something else. Were there a lot of bugs? Was she kicking at the flies under her belly?
Was she sweaty? Young horses tend to be more irritable as they aren't used to itchy tack from sweat or other things and don't like how it feels.
Is she kept in a pasture with other horses? Does she get to socialize with horses on her own terms? If you see no warning, like ear pinning, then what makes you think it is aggression? I think she is just irritated at something. Maybe how the saddle fits or how the blanket feels?
I would completely agree to kicking out in frustration if the ONLY 2 times it had happened hadn't been aimed at a horse. She's never done any sort of misbehaving like bucking/crow hopping other than once last winter when we changed her tack from english to western - just wasn't used to it and needed to "buck it out"! Ever since, her way of misbehaving is tossing her head or scooting forward.
There were pretty much NO bugs on the ride, being early Manitoban October, pretty much everything is dead or hibernating! It was beautiful! But no, I know the difference between swatting at a belly fly with a back leg and a fully extended hind leg kick. My best friend SAW the second incident, she was kicking at the horse behind her. Lucky he was too far away, but it was aimed.
She was tossing her head once she got sweated up, I account that to her ears having sweat on them and the bridal being there, not being able to rub/scratch on anything...big 'ol lesson in too bad, wait til we're stopped! I promise neither of these 2 situations were a harmless shooing of bugs or baby irritations. If it were a tack issue or baby tantrum, it would've happened more than twice and not under identical circumstances to each other.
She is kept with a dozen other horses, both mares and geldings - is far from top dog, but not at the bottom where you'd picture a 3 year old to be. I know she's a socially dominant filly in the pasture for the most part, but how to teach her it's unacceptable under saddle is where I'm stumped. I do think it's limited to big groups, she was in close quarters with 5 other riders a week before this last one and handled herself nicely - got dancy out front, but settled if I kept some distance (15-20 feet). I'm just trying to figure out where this fear of something behind her came from. Maybe she was ambushed by other horses when we moved them to the ranch in May?
I'd also be more inclined to agree with the tack fit if I'd changed something...I do have her going in a different bit than before, but it was changed after the first kick. Makes it inconsistant to the theory. I think taking the winter to "grow up" and maintain the training that's there and then tackling this next spring is my best course of action. I *could* tackle it now, but if it's fear based then her baby brain is just going to see working on it now as forced instead of teaching and accepting. In no way am I trying to put off or avoid this lesson I know she needs, but timing is everything and her more than most is VERY immature for a 3 year old. I'm used to stock horse grades and welsh ponies...they think they know everything by age 2 and Eve barely knows her name at 3! LOL She reminds me of Eeyor some days! Learning to canter was like watching a baby moose find it's legs...she's just a little slower and delayed than every other horse I've worked with. In knowing that, the last thing I want to do is RUSH this and make it into a bad experience for her.
Thank you for all the replies :) very much appreciated!