He decided to allow himself to transform and looking down at his arm he saw his veins throbbing against his skin. He knew he wouldn't be able to change back so with a deep breath pressed the button.
He knew instantly this was a mistake, he felt like he exploded as he fell to the ground in pain he felt his muscles expand and bubble up. He looked down at his cock and saw it violently shaking, he grabbed it trying to stop it but it pulsed in response. He fell backwards and gave in to the transformation, it didn't feel painful but pleasureable. He groaned as his dick ruptured upwards, his arms grew longer and more beefy, his legs thickening in size.
He put his hands against his head as he felt his horns grew larder in size as well as denser. This was the same with his facial features they seemed to stretch and crack, the bones growing stronger underneath his rough leathery skin. He stood up and felt his body increase in mass filling to the entire room. He cried out as he pushed again the walls feeling suffocated. Until the walls gave away like paper, completely obliterating them. He could only see darkness as his mass kept increasing.
He then saw daylight, he was in some sort of town no it was a city. But the transformation didn't end, to his surprise his feets muscles curved do become more animal like, same with his hands. He growled "No...WHATS HAPPENING". He felt a piercing pain in his back as two long leathery rings sprouted out of it. He was now 2 stories high and he could hear muffled screaming around him. He was becoming feral he couldn't stop that. His mind was growing blurry and he could only see two things to hunt and destroy.
The monster in its beast form growled at the humans panicking around it. They were confused and didn't know what it was going to do, they stood there in anticipation waiting for it to make its deadly move.