For starters, what you're dealing with is not a phobia, it's trauma. you were almost killed by a horse and so it's almost certainly PTSD. With far less experience around horses than her friend, I have enough experience to know that a fear of horses is rational for humans because horses are actually really big, and they're the kind of animal you have to have a certain level of caution around so you don't get hurt. That doesn't make them inherently unsafe and so while it's a soft YTA, you're still TA. It's not okay to deny your children of things they show interest in because of your own fears or trauma. And furthermore, while it's valid to be upset your friend did something with your child behind your back knowing you'd not be okay with it (and let's be clear, that's NOT okay), calling her irresponsible when YOU'RE the one with 0 horse experience was uncalled for. If she's a horse girl like you claimand actually owns several of her own horses then she's knowledgeable and responsible enough to have kids around them safely, which we know because she also has a child. Calling someone else irresponsible behind something they're an expert on because YOUR trauma blinds you isn't okay. So it's more of an ESH, because that's still your child and your wishes should be respected plus she lied about it. But two things can be true at once and your reaction was a bit of an overreaction.