Thanos the Mad Titan is one of the powerful beings in the Marvel Universe, whose conquest for dominance has led him to defeat a plethora of Marvel heroes. However, one hero that Thanos has been beaten by is probably the least expected victor imaginable: Squirrel Girl - and it makes perfect sense why the MCU steered clear of the story.
Doreen Allene Green first appeared in Marvel Super-Heroes #8 and was created by Will Murray and Steve Ditko. Squirrel Girl has a long bushy tail, can communicate with squirrels, and has superhuman strength and senses. As ridiculous as the character is on the surface, Squirrel Girl has shown not only staying power, as she starred in the wildly popular 'The Unbeatable Squirrel Girl," by Ryan North and Erica Henderson, but she's also shown the ability to beat Marvel's biggest villains with relative ease. Squirrel Girl defeated Doctor Doom in her debut issue, has taken down MODOK, and has also convinced Galactus to avoid eating Earth. However, her battle against Thanos is what truly sets her apart from other B-tier heroes.
In GLX-Mas Special (2006) by Mark Waid, Matt Haley, George Jeanty, Drew Geraci, and Sotocolor's L. Molinar, the holiday adventures of the Great Lakes Adventures are showcased. Squirrel Girl is working overtime on Christmas eve, helping SHIELD takedown MODOK before Thanos arrives on the scene. The Mad Titan has acquired the "Pyramatrix," an intentionally obvious MacGuffin he intends on using to take over the universe. The hilarious part of this particular story is the battle isn't shown as it happens off-panel, as later in the story it cuts to Doreen dusting off her hands with Uatu the Watcher congratulating her for saving the multiverse, and that "I can confirm that that is, in fact, the one true Thanos, and not a robot, clone, or Simulacrum."
Now, the entire issue is tongue and cheek, but Squirrel Girl adding Thanos to her list of Marvel supervillains she's defeated is well-earned, even if it's not seen in the comic. It's also pretty obvious why this comic arc was never adapted to the MCU. Defeating Thanos in the background of an Avengers film would be unsatisfying and wasteful. Squirrel Girl's character would work best in a fourth-wall-breaking film like Deadpool - not so much in the MCU as it is, even in a funnier movie like Ant-Man. In the comic, the defeat works because of how inherently silly Squirrel Girl and her characterization is.
Hopefully, Marvel brings Doreen to the MCU and she's allowed to kick the butts of some OP villains. Until that happens, readers are left to enjoy her single-handed takedown of Thanos. Silly or not, Squirrel Girl isn't to be messed with.