Shaun was panicking, and you had to shut him up. "Oh my god, what was I doing? Why am I hard! Why are you hard too?!". You quickly adjust the knob on the watch back to where it was.
"Why was.. Oh.. What... Happening.. Oh... God..." Shaun said outloud, the aggitation fading from his voice as it got deeper and deeper. His hands flew up to his forehead as a bony ridge forced its way out above his eyebrows. Shauns pleading questions soon melted into grunts and groans as he lost the ability to speak or process English. His cock began to get hard again, swelling as it grew exponetially hairier, a thick skin sleeve covering it like an animal. Shauns arms got muscular and hung down to his knees as thick kinky hair sprouted all over his rapidly devolving body. The musk coming off him was almost eye watering! Within a few seconds, your former brother stood before you as a hairy, hunch backed, smelly caveman.
Shaun looked at your shirtless body and your cock, and lunged forward. He once again shoved his bearded face into your armpits. He took a deep whiff of your hairy pits and grunted. His huge, hairy cock brushed against your torso, leaving a sticky trail of precum. Shaun then dove down to your crotch and began giving you another blowjob. You moaned and bucked your hips. It felt so good. This was so fucked up, though, but you were enjoying it!
You close your eye and try to at least enjoy the blowjob. You could change your brother back afterwards. For now, having a hairy monkey man fuck you was fun!
A minute or two later, you open your eyes, moaning in pleasure. You look down and gasp! Your previously hairless chest was absolutely covered in a thick rug of brown body hair! Almost as much as your caveman brother. The hair reached down to a crotch that had turned into a kinky jungle. Likewise, your arms were plastered with hair and seemed much longer. A cursory feel of your face confirmed that a ridge was growing their too. And, with the heavy pendulous weight you felt in your sac beneith you, you didnt believe those were still human either! Shit! You were becoming a caveman!
You try to push Shaun off of you, but he was stronger. "Shaun. Must. Make. No. More. Suck.", you grunted, realizing human speech was becoming incredibly difficult. You look down at the watch, almost buried in your forearm hair, and realize that all of the symbols are rapidly losing any meaning!
You realize that if you don't act soon, you could end up as a caveman, and be too stupid to be able to turn yourself back!