When we lived in Sussex my sister had a friend with horses and I learnt to ride on a pony that had been "the fastest pony in Sussex" - in her younger days she'd been a racing pony! She was great till she got it into her head she was in a race, you couldn't really ride her alongside a hedge cos it brought it all back and she was off! If my parents had know how often their 4 year old got run away with!!!
Then they put me on a young old half arab pony that had been gelded late, had an injury and been turned away while still only half broken.... being a little older by then, I got quite good at sliding off him before he broke loose!
As a teenager I was never a terribly confident rider - I wonder why??