the testicles descend.
Even in a litter of pups you can see different rates of development.
Anyone who isn't an animal owner must think we are so strange - my hubby once commented that if I hadn't explained it all to him he'd think we were perverts.
One of the benefits of having my male (dog!)neutered was no more applying sun block to the pink skin on his scrotum to protect from sunburn.
Good luck with the colts - hope you can post some pics at some stage, love to see photos of everyone's pets.
cheers - Dalfyre