It's got unusually high amounts of sugar in it at the moment (hot sunny days, bit of rain overnight - ideal growing conditions) and so more insulin than usual is being produced in their systems to deal with it. This affects some horses more than others. Tiny Pony has a condition called PPID (commonly known as Cushings), which affects about 20% of horses and ponies over the age of 15, and it means they're more prone to laminitis. I suspect the other two have it as well and am going to get them tested this year, but my finances have taken a bashing sorting the fencing out (£1,300) and Senior Dog has incurred a vet bill that's going to be in the region of £400-£600 today thanks to some major dental work, so I need to replenish the savings account first!
The weather is due to break here on Wednesday, we go back to rain, cloudy days and lower temperatures, so that will reduce the sugar levels and he should be fine on it again.