what is this on horses leg??

Yep, ergot. You probably haven't noticed it before as most often they are small, firm, and hard to find unless you're looking, but will get soft and swell when wet. If they bother you, you can peel/twist them free carefully. If you clip a horse's fetlocks, you'll want to make sure they're off before you clip so you don't ruin the clipper blades.

I've had them on every horse I've had-- Arabs, Morgans, all types of stock horses, gaited, etc. I think they just tend to be more obvious on drafts than some other breeds.