Axolotl

The Axolotl is a mysterious all powerful extra-terrestrial entity that takes form of an giant talking Axolotl in Gravity Falls. The Axolotl is far superior in wisdom and power to Bill Cipher, and is estimated to be able to manipulate both time and matter.

Mentions
Journal 3:

On the portal page, Bill Cipher writes a message, stating "I'll get this puppy up and running one day and neither Time Baby nor the Big Frilly know it all will stop me," referring to the Axolotl as "Big Frilly."

On Bill Cipher's page, Bill writes "Curse the Axolotl every conspiracy is true."

When Stanford Pines is stuck in the nightmare dimension, he finds a group of refugees who mention "Praise the Axolotl!"

The Oracle, who save Stanford from the 2-D dimension, is depicted with tapestry containing pictures of the Axolotl.

Gravity Falls:

Bill Cipher states a backwards message when he is defeated, stating "AXOLOTL, my time has come to burn! I invoke the ancient power that I may return!

In this instance, the Axolotl grants them one question, to which Dipper asks "What [does he] know about Bill Cipher?" The Axolotl replies a poem :

"Sixty degrees that come in threes.

Watches from within birch trees.

Saw his own dimension burn.

Misses home and can't return.

Says he's happy. He's a liar.

Blame the arson for the fire.

If he wants to shirk the blame,

He'll have to invoke my name.

One way to absolve his crime.

A different form, a different time."