Thwimp did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):

Thwimp New Render
A Thwimp
Species Origin Thwomp
Alignment Bad
Related Species

Thwimps are smaller, miniature Thwomps that first appear in Super Mario World. Like Thwomps, they are commonly found in castle levels, and pose as obstacles to Mario. They behave somewhat differently from their larger counterparts; instead of staying stationary and falling on Mario when he draws near, Thwimps will hop back and forth, posing an obstacle for Mario to get across.

Aside from remakes and occasional appearances such as Mario vs. Donkey Kong, Thwimps were not seen in Mario games until being reintroduced in New Super Mario Bros. U, where they retain their Super Mario World behavior.