Plessie did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Plessie is a plesiosaur that first appeared in Super Mario 3D World on the Wii U. The name Plessie is a reference to the extinct species, plesiosaur and the Loch Ness Monster which is often referred to as Nessie. Plessie is similar in appearance to Yoshi and is believed to be the same species as Dorrie who looks very similar to Plessie except is blue.
To control Plessie, the player moves the control stick/control pad to the left or right to turn. Moving the control stick/control pad forward will cause Plessie to move faster and move the control stick/control pad backward to slow down. Plessie can also jump if the jump button is pressed.
Super Mario 3D World
Plessiehas similar functions to Dorrie from previous games. Plessie will take players over water as well as other terrain. Plessie appears in five different levels including; World 1-4: Plessie's Plunging Falls, World 5-1: Sunshine Seaside, World -2: Plessie's Downhill Dunes, World -5: Deepwater Dungeon and World -1: Rainbow Run.