"For me?" Jeff eyed the present suspiciously.
"Of course!" Mike laughed heartily. His Santa belly shook with joy. "You've been a good boy after all. And wanting to spread holiday cheer is so important!"
Jeff cautiously tore off the brightly colored wrapping paper. He opened the box to see a harness with some bells on it. "What's this?" he asked as he pulled it out of the box and noticed a bit and bridle as well. However, Jeff had already started to work at getting the present on. His arms easily went through the harness and his face didn't quite fit with the bridle. "It's a bit big don't you think?"
Mike just laughed.
"D-dad?" Jeff stammered. He already felt something off. His hands seemed to cramp and he looked down. "Wh-what's going on?" he asked as he tried to separate his fingers. It was rapidly becoming harder to do as the skin was starting to fuse together. He watched as the four fingers became two and hard black hoof like nails.
"I gave you a present," Mike laughed. "Something that you've wanted."
"What are you talking about!?" Jeff felt his bones start shifting. The heels of his feet rose up as he grew into a more digitigrade state. His feet continue to lengthen and grow bringing him a bit closer to his father's towering 7 foot height. The shoes he was wearing were having a hard to holding together as his feet shifted and changed. Hard hoofs replaced his feet, tearing through the soft fabric. Soft, brown, velvety fur started growing out from his hands and ankles.
Everywhere the fur grew from, Jeff could see an unbelievable amount of muscle grew wherever the fur had touched. It shot up his forearms doubling them in size. His biceps were next to grow as they quickly bulged out into softball sized growths. A deep divot cut into his arms showing off just how defined those muscles really could be. It was the same with his triceps as they bloomed behind his arms with a perfectly cut horseshoe shape. His coat couldn't hold the mass and the sleeves tore away from his body.
Jeff's pants didn't hold on much better as thick calves quickly filled the lower parts of his pants. He heard the loud 'SHRIP' that filled the living room as the muscle and size continued to add onto his body. It didn't help that a bulge grew in his pants as well. Jeff panted as he felt an overwhelming arousal take him over. Sweat started to drip from his brow as he panted over the dull high he felt. The arousal was taking over as his hooflike hands padded at his growing crotch.
Before he knew it, the rest of his pants blew off his body as thick quads filled in where they hadn't been before. The massive rod burst forth and Jeff nearly fell to his knees in pleasure. His dick had grown to almost a foot long with balls twice as large. It was already dripping with cum as his hands desperately started stroking.
"Dad..." Jeff whimpered as he continued to stroke his cock. The soft panting of his breath made it hard to concentrate. He didn't notice as the fur continued to spread up his torso as a cobblestone road of abs filled out his flat stomach. Nor did he see his chest burst out in front of him as thick heavy pecs grew from his chest. With each panting, they jostled and jiggled on his chest. The harness tightened to his chest and the jingle bells started ringing. With each thrust the silence was filled with a dozen bells ringing.
The fur continued up Jeff's neck. He leaned back as it covered his face. All the while, his skull restructured itself. It shifted and changed to better fit the harness. Jeff could feel his nose pressing forward and his nostrils flaring out. His mouth, already open, bit down on the bridle. Two massive horns burst out of his skull adding another foot to his height. He bit down harder as his cock jerked.
A deep neighing filled the room as cum shot out of his massive dick. Load after load pushed out as Jeff fell to his knees. It was an impossible feeling to overcome. Even with the massive amount of power and strength he'd gained over the last few minutes, he couldn't hold himself up.
"Ho! Ho! Ho!" Mike chanted as he watched his son finish in front of him. "I knew you would love it. You always said how much you wanted to help me spread joy. However..." he looked at the throbbing cock, "you can't exactly go out looking like that." He pulled out a large pair of tight red underwear and handed them to Jeff. Jeff put them on, covering his dick.
He took a moment to look around. Jeff knew something was off, but couldn't quite place it. He didn't understand why him being a 6'8" reindeer man who had muscles bodybuilders only dreamed of would be wrong. His hoofed hands ran over his chest and abs and he tried to figure out what was wrong but still couldn't.
Mike clipped something to the back of his harness. "Now. Before we go there's still one more thing we have to do." He did something with the computer and then pulled out his phone. As they walked to the front door, Mike threw the massive bag of presents over his shoulder and headed outside. However, there was no way all of that could fit in the cheap car that he'd been driving. Instead, he pulled out the Chronivac and typed in that the car was a massive sleigh that could easily travel on any surface.
The car shifted and molded to that of a sleigh. It lost it's wheels in favor of two long skis underneath. "Perfect," Mike smiled as he slapped Jeff on the back. "Now, let's get you strapped in." He clipped something onto the back of Jeff's harness and the two were ready to start spreading holiday cheer.