The straps around his chassis were constricting. They were meant to be she told him.
Onyx had a reined bar gag and covering muzzle on that strapped around his horns down to the collar he wore. His chest piece was also connected off the collar and restrained his arms behind him. His wings had special restraints to not damage them but kept them folded to his side. The saddle that the straps for his wings attached to he had on the lower spine was connected by a waist harness and had decorative rope coming down across his lower half's legs and midsection. The rear of the saddle followed his spine back to his tail where his rear legs' straps met to a cover that held a placed plug currently stuffed deep inside him. His spike was stuck on his underbelly leaking by a harness and he couldn't retract it.
Onyx shuttered and gave a whine beneath his muzzle with the binds on his four ankles keeping his legs apart as the plug vibrated inside him. It was torture and when he managed to bend his upper half to a degree, he noted his previous overloads with remnants on the ground of both transfluid and lubricant that had been creeping down his legs. He gave a horse-like pant as his chassis spasmed from the assault on his valve. It didn't take long for him to be sent over the edge. His spike stained his underbelly while the rest of it hit the ground, and his valve gushed similar pushing past the plug.
"Oh, Onyx, look what you've done. Bad boy. You've made a mess," scolded Solus. The centaur's hide instinctively flinched at her touch pleasurably. She ran it across his right side to the front limb which she patted with a smile. "Maybe a walk will get all that excess energy out. Does that sound good while your room gets cleaned?"
Onyx liked walks and slowly nodded, unable to talk with the muzzle. Solus grabbed a rag and helped him clean his parts and legs then winced when she pushed the valve cover down to take out the toy only to replace it with another toy bigger in length but no features otherwise. He grunted and tried to moan as it was inserted. Once it was seated in him entirely, Solus strapped the cover back to keep the toy secure and went in front of him to untie the reins.
He followed her guidance like a pet but twitched his optics feeling each step unsettle the toy. Solus deliberately did this to test him. The beastformer found it hard to walk without wanting to stop and try to get off on it. His struggling persisted when they bridged to the untamed frontier of Torus Heights. They were thinking about putting a conservatory here for its wildlife but for now, it was under his protection and someplace no one had to see him be reduced to a horny mustang.
Solus suddenly mounted the saddle and held the reins while petting his shoulder. "Go on, Onyx, give me a tour."
He treaded through the territory trying so hard not to focus on the toy shifting with every step. If he went faster than a prance, it really got bad. A few times Solus goaded him to trot faster, he only was able to do so for a moment before his legs would start to feel weak and he slowed nearly to a stop to huff and try to hump the air.
"Uh-uh, I said walk. Come on, pet, you were doing so well," she taunted.
His rear legs were bending lower than his front with his torso leaning forward shuttering. He stood still to fight the heavy sensation rattling him until it went away. With a ragged breath through his muzzle, he continued on. It was a test of endurance. He had done it before but not with this size of a toy. It took every bit of willpower to make it to the trail end where he was always allowed to let his body cave without Solus making him endure it again as punishment.
Solus finally said, "Good boy, now overload for me."
Onyx started to trot fast to get the ripples of pleasure go to ravenous waves. He slowed and arched, moving forward a bit before using his front legs to keep from falling and his rear bucked wildly with an assault of heat. The toy was crushing against his walls and moving. His back left leg lifted among humping automatically and stretched among his intense overload that he needed so bad. He almost wanted to cry from how good it was. Solus was just holding on like he was a bucking bronco.
When Solus got off him, Onyx was dead standing with a long growl and leaning forward tired. The orgasm alone drained much of his energy as a whipped pony for his master. He didn't shy away from Solus' touch. Then she whispered something that made him perk up horny again.
"Maybe if you're good tomorrow, I'll let you ride me."
He nearly caused another orgasm from how hard his tail was wagging. It may have been restrictive, but he would lay down his life for Solus, even reduce himself to a pet to be with her.
AN: This was kind of how I figured centaur bondage kind of would work and a little pet play to go with it. Onyx is sometimes the most fun to write for because he has extra features not normally found in other Transformers. I can just go ham on this dude's possibilities, and he really is a slut deep down.