Axolotl

The Axolotl is a powerful extradimensional creature, whose power ranks alongside that of Time Baby. Like Time Baby, he is an enemy of Bill Cipher. He may play a role in any potential return of Bill following his defeat in Weirdmageddon.

He takes the form of a giant talking Axolotl.

Weirdmageddon, Part III
As Bill Cipher is being erased due to Stan's trick in "Weirdmageddon 3: Take Back The Falls/Transcript", his final words are a distorted, reversed message which actually says: "A-X-O-L-O-T-L! My time has come to burn! I invoke the ancient power that I may return!"

This was the first reference to the Axolotl in the TV series itself.

Gravity Falls: Journal 3
Gravity Falls: Journal 3 contained two new references to the Axolotl, hidden in encrypted text:

First, on the portal aperture page, Bill (controlling Ford) wrote: "ILL GET THIS PUPPY UP AND RUNNING ONE DAY AND NEITHER TIME BABY NOR THE BIG FRILLY KNOW IT ALL WILL STOP ME" (Axolotls have their gills on the outside of their heads, giving them a frilled appearance).

Second, throughout the Bill's Ciphers section, there is code also written by Bill which says: "CURSE THE AXOLOTL EVERY CONSPIRACY IS TRUE".

Dipper and Mabel and the Curse of the Time Pirates' Treasure!: Select Your Own Choose-Venture
In "Dipper and Mabel and the Curse of the Time Pirates' Treasure!: Select Your Own Choose-Venture", the Axolotl directly appears to Dipper and Mabel - though this book is explicitly non-canon, as the Axolotl erased their memories at the end.

According to Alex Hirsch, the book is essentially non-canon to the actual series, as it follows a "choose your own adventure" format, having many different possible endings. However, he also said it does contain "one enormous 'canon' secret". It is speculated that the canon secret could be Dipper and Mabel's encounter with the Axolotl in "the time and space between time and space". The Axolotl, a giant talking axolotl, lets each of them ask him a question. Dipper asks him "what [does he] know about Bill Cipher", to which he replies with a poem:This suggests that when, before being erased from reality in the series finale, Bill gives a message that when reversed says "A-X-O-L-O-T-L! My time has come to burn! I invoke the ancient power that I may return!", he is actually invoking the Axolotl for redemption for his crime, and that there is one way to get absolved: he has to take "a different form" in "a different time". This would mean that Bill could return someday in the future, but it's unknown if this will ever occurred or not.

The line about "sixty degrees that come in threes" apparently refers to the fact that Bill's shape is an equilateral triangle: three sets of sixty degree angles (adding up to 180 degrees).