The Horrifying Sweaty One-Armed Monstrosity: was Louis CK, re-voiced by Alex Hirsch

https://en.wikipedia.org/wiki/Louis_C.K.#2015%E2%80%932018:_Sexual_misconduct_revelations
___

Also: https://en.wikipedia.org/wiki/Gravity_Falls#Broadcast_edits

The symbol on Grunkle Stan's fez was changed from a crescent shape resembling the Islamic crescent to a fish-like symbol mid-way through the first season's broadcast. The symbol represents his membership in the Royal Order of the Holy Mackerel. When the series was put on Disney+, the crescent-shaped symbol was edited out entirely, leaving a symbol-less fez in the early episodes—later episodes featuring the fish-like symbol were unaffected. However, the crescent symbol remains in the thumbnails, and on the zodiac wheel in the title sequence. Hirsch drew attention to the change on Twitter. Disney has not commented on why it was removed.[32]