Jackie and the Pony Trekkers
The Jackie and Misty books are about a 14-year old girl who a pony in a magazine contest and they get up to all sorts of adventures! Once you start reading it's really hard to put down!
Jackie Won A Pony - the first Jackie and Misty book.
Jackie wins a magazine contest and gets to choose out her very own pony, but instead of choosing one of posh ones from some stables, she find a cart pony and that trots off from it's owner to meet her and this where the adventures begin!!!
Trust me, these are the best books EVER!! They are very old so you probably won't find them in proper book shops. Just search the internet!
Note from Sydney: Is this the series you were talking about? I had to dig around a bit to find it...!