Fur, Scales, Claws, and Tails (A Monster Falls AU) Book 1 in the Portal series.

He had reached a calm section, so he doggy-paddled his way over to the shore. As he climbed out, he took off his hat and rung it out.
"Hey Pine Tree! Nice of me to push you into the river, huh?" A cold, dangerous voice echoed around the clearing.
"Bill Cypher?" Dipper's squeaky voice asked with more than a hint of fear, "You're gone. W-We defeated y-you!"
"Not quite you haven't! Remember, reality is an illusion, the universe is a hologram, buy gold, BYE!" Bill disappeared in a puff of smoke.

Disclaimers
I do not own Gravity Falls or the characters in Gravity Falls or Reverse Falls
I do not own the cover art. I'm pretty bad with digital art, so congrats to the person who made it!

Claimers
I do own the characters from the non-Gravity Falls dimension. (It's the one where no one has heard of gravity falls.)
All art is mine unless stated otherwise.