Green grass has estrogen, which causes fat deposits and adds to the problem by increasing body weight and causing histamine release resulting in laminitis. The typical picture is a big, fat, cresty horse with the hind legs tucked under to get the weight off the front feet. Most of the body weight is in front, so when the horse tries to move one foot the other hurts so bad he give the appearance of walking on eggs. The legs are not involved except that the flexor tendon sometimes can fatigue and may have some swelling. Acute or sudden cases of founder, where there is a specific incident or just a sudden severe lameness, are emergency situations.
Veterinary attention is necessary.
If treatment can decrease the reaction in the foot to save the lamina, it may mean the difference between having a salvageable horse or one requiring euthanasia. Many foundered horses can be managed so they can again become useful, but many also end up chronically lame and some are affected to the degree they must be humanely destroyed. Some of those big, fat, easy keepers, just cannot be on pasture. Some will founder just eating the weeds and grass they can reach through the corral fences. The other thing to remember is - all things in moderation.