Your hand grabs on to a strap of some sort and you wondered what it could be about. You pull your hand out to see yourself holding something that looked like hairy peaches with a short tail sticking out. You recognize the animal it comes from unfortunately and regretted the thought that you'd try the first thing you pulled out.
Despite your annoyance of having to strap on a monkeys naked hairy butt, you put the strap through your belt loops, and buckle it up. You see a full view mirror and decided to see how it looked. It was the most ridiculous thing you've seen. You were so thankful you were alone. But you were still amazed by how real the thing looked. The rubber like skin with little hairs on it really brought the attention to detail.
You had enough of looking at the monkey crack and decided to take it off. However, the belt loop you put the strap through seemed to have disappeared. But the thing was still on your pants. How was it holding up. You try pulling it off until you felt a tingling in your own butt. Your eyes shoot wide open as you feel your cheeks grow in size. You fall to your knees from the pressure as your pants started to give out to your bulging butt. You hear a ripping sound as your large posterior burst out the back of your pants, leaving a big tear in the seat area.
Your pants were no concern at the point as you felt your butt growing more. You didn't notice, but your butt was actually filling the empty space for the baboon butt you wore. The skin of the costume piece fused with your skin creating a seamless connection with your body. You moan as your butt not only completely filled the empty space, but grew even bigger.
You give in to the urges and start rubbing your posterior. You could actually feel your hand rubbing against the rubbery skin with the hairs standing up. You also feel your lower back jolt as the tail starts twitching on its own. You can practically feel your spine extending our to accommodate the appendage as it falls in between your crack. Your finally relived of the pressure as you let go of your butt and pant in exhaustion.
After a few seconds to recover, you stand up and look over your shoulder to see the full view mirror. You see the baboon butt seamlessly attached to your hindquarters as the slim tail sways around. This was now your butt. You didn't know anything like this could happen. You didn't know how to even react. On one hand, you looked like a freak and would either get made fun of by everyone, or get sent to a lab for experiments. On the other, your butt never felt so good in your life. The whole experience of your butt growing as well as the tail is some of the best experience you ever had. And even now, your bulbous butt hanging out of your pants felt... Somehow pleasing in a way. You couldn't believe that one point you were disgusted with putting it on as a costume, but when it was truly yours you were almost proud of it.
You look in the mirror a few more minutes to process everything that happened before you think of how you deal with this.