Piranha Plant is the first DLC character in Ultimate.
Super Smash Bros. Ultimate is actually the first game ever to have Piranha Plant as a playable character (although Petey Piranha is playable in the Mario spinoffs).
Piranha Plant is considered to be one of the most unexpected and controversial inclusions as a Smash fighter: despite being an iconic Mario minion.
This is one of the few depictions of a normal Piranha Plant having feet. Most depictions of the Piranha Plant are implied to be rooted to the ground, their pipe or their flower pot. The Ptooie along with Stalking Piranha Plant actually does have feet: but it's unconfirmed if they are identical to regular Piranha Plants or if they are subspecies. Ptooies and Stalking Piranha Plants are also not depicted with a flower pot.
Ptooies also puff spike balls into the air in a manner similar to the Smash Piranha Plant.
Piranha Plant changes its coloration into that of a Putrid Piranha when using its poison attacks.
Piranha Plant's extremely elastic neck is possibly a reference to the Stalking Piranha Plant or the Piranha Creeper.
Each of Piranha Plant's (currently confirmed) 7 alternate skins are a reference to Piranha Plant subspecies. 
The 1985 Piranha Plant skin has the classical green Warp Pipe. Half of these skins have a warp pipe instead of a pot: and these pipes use a completely different model as the pot (most noticeably missing the holes that the pot has).
As Piranha Plant's trailer showed; an 8-bit Piranha Plant is also a stage hazard in Mushroom Kingdom.