"America's Got Talent" star Zac Gordon ... aka Horse ... aka the guy with iron testicles ... is lucky to be alive ... after smashing his car into a tree while trying to avoid striking a deer, TMZ has learned.

Horse -- who became famous for allowing people to bash him in the scrotal-region on AGT -- tells us he was driving in Harrisburg, PA Saturday when a deer suddenly ran out in front of his car.  Horse says he jerked the wheel, trying not to kill the animal ... but lost control and went careening into a tree.

Zac says he hit the tree so hard the car flipped over -- "I legitimately thought I was going to die."

An ambulance rushed to the scene and hauled the AGT star off to the hospital. He was released later that night with a possible concussion, sore neck and scratches ... but luckily no major injuries.

"I'm just glad to be alive," Horse tell us.

Now, here's Zac crashing into other things:

The nut-smashing "America's Got Talent" star who became famous for abusing his testicles on TV was seriously injured in a stunt-gone-wrong ... suffering several broken bones and a soiled pair of underpants, TMZ has learned.

The man at the center of the painful situation appeared on the show as "The Horse" -- real name Zac Gordon.

Gordon tells us he was filming a stunt last weekend with some of his "AGT" buddies in which he was supposed to jump from a two-story roof and land crotch-first on a wooden plank.

In the video ... you see Horse hit the plank -- but he says he ultimately landed on a bucket of cement that was being used to hold the wood in place.

Horse tells us the impact was so intense ... he broke several ribs and defecated in his shorts upon impact. 

The amateur daredevil says he refused to go to the hospital ... but consulted with a doctor afterward just to be safe. 

Don't worry America -- the guy who got his nads bashed about a million times on "America's Got Talent" this week, tells TMZ ... his family jewels are totally fine.

In case you missed it ... Zac Gordon -- known as "The Horse" -- shocked the audience with his talent when he let people KICK HIM IN THE NUTS ... break boards on his nuts AND smash a cinder block with a sledgehammer ... on his nuts!!!!

Now Horse assures TMZ ... contrary to how painful his act looked his balls suffered no injuries and tells us, "No cup! I've been doing this 10 years and I'm healthy as a horse!"

Gordon also tells us he's previously consulted with his uncle -- who is a doctor -- and his "talent" is not a medical concern.

Good thing because Howard Stern, Howie Mandel and Sharon Osbourne all said yes to his nut-smashing skills ... sending him through to the next round, where he's sure to take another beating.

Balls. Of. Steel.