A Thwomp is an enemy in most Super Mario games, usually in Bowser Castle levels. If Mario ever got near them, they would come crashing down and squash him, making him lose a life.
In the Mario Kart series, Thwomps are obstacles that usually appear in Bowser's Castle tracks too (a few exceptions). They can either block a racer's path or squish them, which in turns makes them lose lot's of time.
Thwomps appeared in every Bowser Castle track. These Thwomps are the first to not feature spikes. However, they become active only on the second lap. Super Thwomps appeared on Rainbow Road (SNES). Unlike the ordinary gray, frowning stones, these shine blue and white. They also activate after the first lap. These unique Thwomps cause racers to spin out on contact instead of being crushed, as if by a Star.
Mario Kart 64's Thwomps were based on their appearances in Super Mario 64, and were only seen in one course, being Bowser's Castle. These Thwomps are blue, more cubical, and still lack spikes. They could make racers going at a high enough speed spin out on contact. A few of them in a corridor move from side to side in order to block racers.
In a closed off jail section, a green Thwomp (nicknamed Marty) can be seen, the evil laughter throughout the Thwomp hallways is thought to be from him.
The Thwomps in this game behaved similarly to the way they did in Super Mario Kart. They appear in all four Bowser Castle tracks. And, they make you lose a stunning 8 Coins when squashed. This is the last game to have the old, spikeless Thwomps.
Thwomps appear in Bowser's Castle. They are much bigger than their previous incarnations, and now, these are the standard designed Thwomps that are still being used today.
This also marks the first time that Thwomps cannot be destroyed by a Star. In the 3 previous games, they were not indestructible.
Thwomps appear in Bowser's Castle once again. Returning is a Thwomp that moves from side to side in a secret hallway, like the ones in Mario Kart 64. Thwomps are also featured in the DS version of Bowser Castle 2, but they are like the new ones now.Bowser's Castle, the Nintendo 64 version of Bowser's Castle (Marty is no longer green), and the GBA version of Bowser Castle 3. A ginormous Thwomp that creates shockwaves in the dunes appears in the center of the Battle Stage Thwomp Desert along with some regular sized ones that are buried in the sand. You can do Tricks off of those.
Thwomps return in Mario Kart 7, in both the new Bowser's Castle and GBA Bowser Castle 1. Also, the retro version of Rainbow Road (SNES) includes the long missed Super Thwomp, which now take on a look of regular Thwomps but are sparkling white in color. They still shock drivers, and as an added feature, they make little waves in the track that racers can perform Tricks on.
Thwomps reappear in Mario Kart 8 in the course Thwomp Ruins, their first track dedicated to them (excluding Thwomp Desert, because it was a Battle Stage). Unfortunately, they fail to appear in the new Bowser's Castle, the first time in the series. Once again (for the fourth time [Mario Kart: Super Circuit released a retro of this course without the Thwomps]), Rainbow Road (SNES) comes back again, along with the Super Thwomps (they still look like the ones on Mario Kart 7).