Warning: This post contains spoilers for Marvel's What If...? episode 3.
Hank Pym is known for being the creator of the Ant-Man technology, and the third episode of Hank Pym.
Scott Lang (Paul Rudd) and Hank Pym (Michael Douglas) made their MCU debut in Peyton Reed’s What If…? episode 3 changed that and made him a villain, which also happened in the comics.
What If…? episode 3 explores what would have happened if the Avengers had been mysteriously killed one by one, thus leaving Nick Fury without the team that would protect Earth from all types of threats. The one responsible for the deaths of the would-have-been Avengers turned out to be Hank Pym, who had a falling out with S.H.I.E.L.D. after his daughter, Hope van Dyne, died during a mission, so he wanted to avenge her death by killing all of Fury’s prospects for the Avenger Initiative. For that, Hank Pym used the Yellowjacket suit and took on that villainous mantle, which also happened in the comics but under different circumstances.
One day, while working in his lab, Pym accidentally dropped some vials containing various chemicals that once he inhaled induced changes in his mind, manifesting as a personality crisis. These side-effects led him to take What If…?.
As mentioned above, Yellowjacket hasn’t appeared in other MCU movies but his helmet (a giant version of it) Ant-Man and the Wasp: Quantumania, though it's unknown what his role will be this time.
Marvel's What If...? releases every Wednesday on Disney+.