Dowmentio is a boss who is a combination of Dimentio and Dowser. He only appeared in Part 23 of Into the Fourth Dimension. Dimentio combined with Dowser (who agreed with Dimentio's plan) to form Dowmentio, who Mario then fought. Mario spun Dowmentio by his tail like he did with Bowser and threw him into two bombs, but then Mario was defeated, and Dowmentio took the D-Stars and almost used their power to destroy the home dimension with the Mushroom Kingdom in it, but Mario took a stand and finished off Dowmentio by throwing him into the third bomb. He then split back in Dowser and Dimentio, with the Mega D-Star (all 120 D-Stars combined) being released. Dowser was blasted off, but Dimentio took another stand against Mario and his friends.
- Dowmentio is yet another counterpart of Bowser just like Mastar, Cosmitalitaurus, Zythurvion, Dark Bowser, Bowser X and Dark Bowser X.
- Dowmentio is the second Bowser in MarioMario54321's videos to be a combination of two or more other beings, the first one being Dark Bowser X.
- Dowmentio is a super-version of Dimentio just like Super Dimentio.
- Dowmentio is supposedly the final boss of Into the Fourth Dimension, even though the series continues with Dimentio after he is terminated.
- Dowmentio was fought in the Super Mario Star Road ROM and is the first Bowser Counterpart in MarioMario54321's videos to be fought in a ROM other than the Super Mario 64 ROM, other than Dark Bowser X who was briefly fought in the SM64: The Missing Stars ROM when Mario became Fire Mario X.