The best recommendation we have on the ideal weight of any horse or pony is to evaluate its body condition score. If the animal is prone to laminitis, you certainly do not want it to be too fat. Most recommend the body condition score to be around 5 and certainly no higher than 6. HorseQuest has a piece on how to determine body condition score. It has examples of the various condition scores, as well as descriptions of what an animal should look like at each score. It can be accessed by clicking here.