Dr Mario World : He's a Doctor now

Since Doctor Mario World, Bowser is the only one who's actually a real medical Doctor, unlike the others mad scientists. Now, he might be a member of their Evil Doctors Mastermind club now.
Fun fact, Bowser meet all of them at least once ! Eggman in Mario and Sonic, Wily in Smash Bros Ultimate, and Cortex in Skylanders Imaginator ( only on switch, though )

(  By the way, becoming a doctor seems really simple in Mario's world, since almost everyone can change their initial job to this profession. )

Bowser, Eggman, Wily and Cortex (c) Nintendo, SEGA, Capcom and Naughty dog respectively.