Adventure time: Pb's Invention

The Animal

FINALLY AN UPDATE! Sorry, i've been very busy with another fanfic i'm writing. But here you go!
Finn's Pov
~~~~~~~~~~~
Pb smiled sadly as she brought out my new arm. It was like a robot arm- metal and had a metal C and the end for a "hand". I sighed.
"Thanks,Pb," I say. She nodded as she latched it onto my shoulder. I moved it around and it nearly pinched me. "It'll take a while to get used to," she said. "I know," I replied. She looked like she might cry. I immediately regret being so annoyed with her. She was just trying to help. But right now I was too tired to apologize. I met Jake, Fionna, and Cake at the tree fort. They all smiled when I walked in. "Looking rad!" Fionna said.
Cake and Jake just ran up and hugged me. I tried to smile. We played video games the rest of the day, but because of my new arm I couldn't play. When we were about to go to sleep, we heard a loud roar and then someone crying for help. I looked out the window and saw an animal that I have never seen before. It had three heads-one a bear, another a lion, and the third a snake. It had huge black wings and talons as long as an elephants trunk. The beast had poison dripping from its tallness and smelled like gasoline. I grabbed a sword and shield and ran outside. I saw a small girl laying under the beast, shaking and crying. "HEY!" I yelled. The animal turned and charged towards me. The girl scrambled away and Cake grabbed her by her arms. They disappeared inside the tree fort, leaving me and Fionna face-to-face with the hideous animal. Jake was nowhere to be seen. We started to try and stab it, but its scales were like iron. Then, out of nowhere, a double-bladed axe flew out of the air and into the lion's mouth. The creature collapsed, dripping silver blood from its mouth(s). A figure with long black hair and a no-smoking shirt appeared, grinning wickedly. "Marceline?" I asked. She nodded and flew down to get her bass guitar/family axe. She pulled it out and it was covered in silver goo. "Yuck!" she exclaimed. Me and Fionna just laughed, surprising me. I haven't laughed in 2 days since my arm was replaced. "How did you know to come?" I asked.
"Jake and Pb came and got me." Then, Jake walked up with Pb on his back. "Finn! Fionna! Are you guys okay? That [insert random monster name here] almost killed you!" "Yeah," Fionna said with confidence, "We're fine!" She smiled that confident smile that I wish I had. If only my arm was still here.......