Explaining Quadrupled Koopa Troopas
A thing people get thrown off by is how sometimes Koopas like in Super Mario Galaxy 1 & 2 are walking on all 4 instead of 2 legs. However, quadrupled Koopas have been a thing for awhile and only occasionally show up. Since Mario Bros. Koopa Troopas or Shell Creepers as they were called then were quadrupled, and this is a detail that continued into Super Mario Bros. and Super Mario Bros. 3. While it may not be obvious in the Sprites in the games unless you pay attention. In addition the art for these games have them going on all fours.
It wasn't until Super Mario Bros. 4 Super Mario World did they become bipedal. From here Koopa Troopas would mainly remain bipedal, but occasionally would go bipedal here and there such as the Galaxy games, or in the RPG games like Mario & Luigi: Superstar Saga with the Troopeas.
My hypothesis is that all Koopas are bipedal including the quadrupled ones, but are forced onto all fours because their shells are too heavy. My evidence for this is in Mario Bros. the Shell Creepers for a brief moment go bipedal, it happens when they exit their shells to flip it back over and then re-enter it.
Follow-up Written by Eris the Lorekeeper