Thing's venom upgrade finally turns him into a real monster
When the Thing turns into Venom in an Alternative Marvel Universe Story, the Hero Becomes a Horrific Monster Who Represents Ben Grimm's Worst Nightmare

Ben Grimm's The Thing has long considered himself a monster as he transforms from a human into a rocky creature, while the hero of the Fantastic Four transforms into a truly terrifying monster when he transforms into Venom. In Marvel Comics' Peter Parker: The Spectacular Spider-Man #304, during the events of Secret Wars, it was revealed that Green's world was infected by a symbiote opposing Spider-Man, as The Thing assumed his darkest form ever.
In Peter Parker: Spectacular Spider-Man #304 by Chip Zdarsky, Adam Kubert, Jason Keith, and Travis Lanham from Marvel Comics, the titular hero ends up in an entirely new timeline, where Norman Osborn Changing his "Goblin Formula" into a health supplement helped him win the public's trust. Norman would back Harry Osborn's presidential bid, and when he was elected as Commander-in-Chief, he enacted a totalitarian rule where the superheroes - including Captain America - who weren't killed or turned into his army were forced into hiding, forming an Underground resistance. When Spider-Man frees Doctor Doom from prison and helps him return to his own timeline, Osborn unleashes one of his greatest soldiers: a symbiote-infected Ben Green who becomes a completely different people A kind of monster.
Famously, Ben Grimm drew the end of the stick when talking about the transformation of the Fantastic Four, as Reed Richards gained his stretching abilities, Sue Storm could turn invisible, and Johnny Storm transformed into the Human Torch, and Grimm's appearance completely changed as he became a rock-like monster who couldn't transform back into human form. His rugged appearance caused Green a severe trauma, but in the end, he accepted being a weirdo--though he still struggled with losing his human form.
Ben Grimm's Monstrous Venom Form Is Terrifying

Grimm's Venom form eventually became a small part of the Resistance. He battles Spider-Man, Captain America, and Doctor Strange at the same time, as his hulking frame and symbiote power hold the heroes back before he can be overwhelmed and defeated. Green taking Spider-Man's place and gaining the symbiotic suit in Secret Wars suggests that the MCU may be in greater danger if events unfold as they do in an alternate reality. Pointy shoulder armor, a pouch enough to impress Rob Liefeld, and his combo-rock symbiosis abilities, it's The Thing in its scariest form ever.
So, while Ben Grimm of Earth-616 may think he's a monster, his The symbiote that served as Venom in an alternate reality showed what the real monster would look like. Thing becoming Venom creates a powerful, new villain version of the hero that almost becomes the end of the one-man Avengers. Fortunately, the heroes prevailed over The Thing, though the surprising new Venom put up a fierce fight before falling.