Can you keep her in a smaller paddock with minimal grass?
It could be next to where the thoroughbred is and they'd still have company. You could have the thoroughbred graze out a small paddock then move him to a new one right next to it, and put the pony into the grazed out paddock. The much reduced pasture would benefit the pony and the new grass would benefit the thoroughbred.
I have a stockhorse gelding that I do this with for this same reaon. He could get fat on a sniff of grass!
Good luck!

This is a tricky problem and I sympathise. I have a pony who also puts on weight easily but luckilhas not developed any side effects yes as I am a bit obsessive about managing it (wish I could manage my own as well, serious case of "if I can't fix me I'll fix my horse going on!!). There has been soem interesting research with grazing muzzles and the general feeling is they can actually make things worse because the horse knows it is limited and then when he gets the mask off he will then "binge". I find letting them graze as normally as possible but with other "eaters" to limit the actual amount. Sheep are a good mix (if you have access) - added bonus of grazing with sheep is that the sheep eat the horse worm larvae (doesn't harm the sheep) before the horse reingests them. Mine is a little 11 hand pony but he soemtimes shares with big shires and a few others and this means he is not chomping. In the spring, I will put him in a smaller paddock (not a "starvation" paddock, just much smaller), again with others. I try to mimic his wild pattern (he is a welsh mountain pony) so I don't give him extra feed at all through the winter unless its really necessary so that by spring he is pretty lean (a concept that "fat pony" owners find difficult to bear!) - he will then fatten up a bit through the spring and summer (but I still keep an eye on it) and then he is ready for the winter again. In addition, bear i mind that in a smaller paddock he is not having to walk far for his food so he is no getting any exercise so a few longlining/riding sessions/carriage pulling or whatever your horse does will hep to burn off some of the extra weight. Hope that helps a little :-)

Some good advice given her by Vicci - it is a hard thing to manage, I had a native british Fell pony and they get fat on the slightest whiff of fresh green grass. You could try a bare paddock or large pen of some kind, and a haynet with really tiny holes instead of a grazing muzzle. They have to work hard to pull small amounts of hay out of the holes and it takes them ages to finish it. If you dotted several nets around the pen, each with a small amount of hay in she would at least have to walk around a bit as well.

Had the vet evaluate Sraddha ,my Connemara pony today.He said she is fat but not obese. Advised using the grazing muzzle all day except for twice daily feeding of pannikan hay,which she is now eating.He said it takes 10 days or so for them to accept a new food.The Speedi beet she will eat when she gets hungry enough hopefully.To feed her in the round yard so she is not tempted to eat the lush grass after Queenslands ( Australia) flooding rains, full of sugars at present.He said two days of temps in the 30 s should burn off the sugar.She has no signs of founder , no fat pads on her rump etc.She hasn't increased the hole any further ,it is rubber ,so he advised putting leather in situ there.When the grasses are back to normal, continue to feed her the hay.He will consult a vet nutritionist and advise me of a simple diet .Exercise of course is imperative which ,now the weather has settle a bit I can continue.It has been tooo hot or for the past 4 days tooo wet. I hate putting the "Hannibal Lecter " muzzle on but guess what ,she hates it too.
So thank you all for helping Sraddha,she really is a trick.Isn ' t this forum brilliant! I will keep you informed, we are all on a learning curve, and always will be.Sadhu!

The vet also said Connemara's are the least likely breed to founder.So go get a Connemara folks,she has the sweetest temperament!