Avengers: Endgame Trivia #94: As we reach closer to hit the century, we bring you a bit of history about Groot which is now voiced by the mighty Vin Diesel. Groot was this big friendly tree seen in Guardians Of The Galaxy. He sacrificed himself and took one for the team in part 1.
Though he returned as Baby Groot who was loved by everyone for its charm. But, Groot wasn’t always with the Avengers. He is Stan Lee’s one of the earliest creations and made his first appearance in 1960 comic.
Stan co-created Groot along with Jack Kirby and he was this monster back then. According to CBR, “Adam Warlock’s not the only cosmic player with surprising ties to Stan ‘The Man.’ A full year before the birth of the modern Marvel Universe in 1961’s ‘Fantastic Four #1’, Stan Lee and Jack Kirby created a monster that would go on to become one of the company’s surprise breakout characters in the 2010s.”
It also states: “The walking, talking tree named Groot debuted in a story in a 1960 issue of the anthology series ‘Tales to Astonish’ of course, he was a bit different than the Groot you know now. He started off that issue saying ‘Fool — none can withstand the mighty Groot! You are doomed! You and your town shall perish!’ That’s a bit more complex than ‘I am Groot.’ At the time, Groot was just one of many done-in-one monsters churned out for proto-Marvel’s horror-filled anthology titles. He wasn’t meant to make a comeback, but he managed to pop up once a decade until he was revamped in 2006’s ‘Nick Fury’s Howling Commandos’ series. After that, Groot joined the Guardians and became the character fans love today.”
Also, according to a theory on Reddit, “Guardian of the Galaxy’s Groot was one of Stan Lee’s first creations at Marvel (and originally a villain) predating Spider-Man and most of the Avengers, including the Hulk, Iron Man, and Thor.”