Sarcasti-Pup is a comic book character from the comic book of the same name. It's also a parody of a famous cat character.
It's unknown what's the lore or history of Sarcasti-Pup, but it is implied to be the same as the character he's based after and it's implied he's a beloved and popular comic book character in the universe of the series, giving how Stan and Soos recognize him from the get-go.
Sarcasti-Pup only appears in "Gravity Falls: Lost Legends" in the tale "Comix It Up!". He appears for a very brief scene during the climax , where he wanders around the comic of Lil' Stanley, Stan attempts to get him to buy some Lil' Stanley comics, but he only thinks in how he's indifferent to Tuesdays which causes Stan to yell him that he'll poison his lasagna then.
Later when Soos discovers that Stan is a comic book lover like him, Stan tries to avoid him from giving him a hug, claiming that Sarcasti-Pup could make fun of him, since his thought bubbles can be both sassy and hurtful.
He's a yellow anthropomorphic dog that seems to be a Labrador, with a bushy tail, he wears an orange t-shirt that says "SASS", he wears a deadpan expression. Since Sarcasti-Pup is a parody of Garfield he is drawn in the style of Jim Davis.
- Gravity Falls: Lost Legends ("Comix It Up!")
"I am indifferent to Tuesdays."
- He's a parody of Garfield. They have a lot of common, like their love for lasagna, the fact they only speak in thought bubbles, their dislike for a day of the week and their dry, sassy thoughts. They're even drawn in the style of Jim Davis. Although, unlike him, Sarcasti-Pup is indifferent to Tuesdays, while Garfield hates Mondays.