"It" won't let me hit the "like" button, so ditto taking over the disciplinary duties that mom is being so lax in.
I bought my now 18 yr old when he was 2-1/2. I don't know anything about him before I bought him but the general concensus among breeders is that his mom was a push over and never corrected him for anything.
He is oat/corn/soy intolerant and also has enivronmental allergies so adjusting his diet for these things helped a lot. However, he will be a snotfaced brat until he takes his last breath.
He is a pushy bullying type of horse when he's being bad; he is the sweetest most loving fella when he's being good.
He is very intelligent and sometimes makes the mistake of showing me just how intelligent he is because that's just about when I tear into him for some piece of foolishness he's trying to get away with.
I'm sure his gene pool also played a huge part in attitude as I know more about his blood line than I do about his birthing and how his dam raised him.
I could write a book on his life-long antics (which are precisely why I've kept him, elst he'd probably be dead by now
My horse was 2-1/2 when I got him, nearly 16H and fully broke to ride

I know effort went into his manners but, I don't think whomever trained him looked too deep into what makes this horse tick - lol lol lol
It could be, by the time my horse hit 2-1/2 years, it was too late to modify his "agenda" which would always be to try and pull one over on anybody or anything not paying attention. On the plus side, he is great about chasing dogs out of the pasture, no matter how big/barky/growling they are
My point to all that is, get onto that foal right now, right now, right now and teach it who the alpha is