Fat pony , needs to lose weight !!?

You should start by cutting back the amount of grain she's getting. As long as her hay isn't too rich, it should have a big impact on her weight, so she could probably have as much as she wants. Get out the lunge line and start by lunging her for half an hour 6 times a week. Don't work her too hard until she gets more in shape, you don't want her to get sore or become sour, so use good judgement with the amount of trotting and cantering you do. It also wouldn't hurt to hop on her bareback and take her for a little ride if she's safe to ride bareback. As she becomes more fit, start working her longer and harder, just try avoid over working her, and make sure she gets one or two days off a week to rest.