I don't have the answer, but I think that you can compare it to humans: if you switch jobs and suddenly become part of an environment where people jokingly swear a lot, a bit of a rowdy bunch, you will start to adapt to that and probably also take part of that attitude (maybe a bit more assertive?) to the other parts of your world (friends, family etc.). But then you go to church on sunday and you're automatically at your best behavior again, right up to monday morning when you're back on the job with your mates.
So now I think about it, I do think that your environment influences how you act and respond, but at the same time you're the one who decides how to act in each situation.
And two boysterous stallions doesn't necessarily mean a high-spirited mare. She could also become very sour (being pestered all the time), or dull (if you ignore them they will move away sooner), and much more.
By the way; do you mean the big British pony breeds when you say native pony? Because if I look at native british ponies I see a huge array of breeds and characters; from very feisty or very slow Welsh Cobs to dull shetlands and hyperactive irish cobs. Breed and character aren't the same: Blacky and Sjors are both Shetlands, but their characters are miles apart.
(they both like to kick each other though
New horse book: Mandala horses!
Never stop making mistakes! Natural Dressage