im not an expert..but from what I understand and had herd from a few horse loving friends that have been around horses and baby horses all there lives the weight limit for any horse is half there size...so if it is a horse that is 1000lbs then the weight limit total with tack,saddle,rider,etc would have to be around 500lbs....byt like I said im not an expert...maybe try and walk into a local vet office and ask them..good luck!
Oh jesus, half their size? That is grossly over estimated! If you put a 450lb person and a 50lb saddle on a 1000lb horse, it would be under EXTREME strain. Especially a young horse.
Like someone else said, 20% of their weight. For an 800lb horse, it would be about 160 lbs, and that includes tack and rider.
Then again, it all depends on how proficient the rider is. I'd rather see 200lbs of excellent balance that can move with the horse and not slam on their back, than even 120 lbs of horrible balance that moves against the horse and wrenches around and balances on their mouth.
Either way, both of the young horses that were posted on here still *look* very young and baby-ish. I wouldn't ride either of them until they evened out. Like one person said, while the knees close up between 2 - 3, that is the least of your worries with a young horse.
Also, like it was mentioned that horses can practically grow two inches in a week, it puts strain on the horse. Just like a gawky kid can have growing pains, it hurts to grow that fast. I wouldn't ride a horse while they are growing so sporadically, either. Depending on the breed, between 2.5 and 3.5 they even out and their growth happens more overall, instead of teetering between butt high and whither high.