Both in- and out-of-universe, Axolotls are a reoccurring element in Gravity Falls.
In the first episode and the online game Mystery Shack Mystery, the Mystery Shack tank contains what seems to be an Axolotl. However, in all subsequent episodes, the tank is empty.
In a reddit AMA, series creator Alex Hirsch said this:
SPOILER QUESTIONS will either be ignored or responded to with an image of an axlotl (Sic), the albino frilled Mexican salamander.
In the series finale, "AXOLOTL" is used as a keyword and
last words are
[Reversed] A-X-O-L-O-T-L. MY TIME HAS COME TO BURN. I INVOKE THE ANCIENT POWER THAT I MAY RETURN."
Finally, in Gravity Falls: Journal 3, there are two references to axolotls.
One passage contains this line:
The creatures cheered me, shouting "Praise the Axolotl!" (I have no idea what that means)...
Additionally, the sketch for "The Oracle" has a tapestry in the background with three Axolotls on it.
Is there any deeper meaning to this, or Hirsch just messing with the fans?