Spits out a cloud of poison in front of it, dealing damage with seemingly no knockback or flinching. Piranha Plant changes its color to match the Putrid Piranha enemy from the Paper Mario series when executed.
Retreats into its pot and pauses briefly before stretching out to bite an opponent. The pot can be tilted in order to aim the attack. It possesses a form of armor while inside the pot, shrugging off various attacks. Likely a reference to Elasto-Piranhas or Stalking Piranha Plants.
Mario runs towards Piranha Plant's pot, only for Piranha Plant to pop out of the pot and bite Mario, sending him flying, before smiling.
Bites the air around it three times, before smiling.
Piranha Plant is the first DLC character in Ultimate, the second being Joker.
Piranha Plant is the only DLC character that is free (for a limited time before being purchasable on February 1st).
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.
The Smash Piranha Plant makes references and shares the traits of multiple Piranha Plant subspecies.
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, Stalking Piranha Plant and Elasto-Piranha Plant actually do 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: although the Elasto-Piranha Plant does have a warp pipe that it wears as pants and can still walk with its feet.
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, the Piranha Creeper or the Elasto-Piranha Plant.
Piranha Plant's up special is a reference to the Jumping Piranha Plant: which can also use its leaves to hover.
Each of Piranha Plant's 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.
As of Update 2.0.0: If the player tries to play as Piranha Plant in All-Star Mode, the save data of the game may get corrupted. However if the Player has an Nintendo Switch Online Membership, he/she can fix this through the Save Data Cloud Back Up. However, this was patched in a later update.
Also, when it is the target of the Smash taunt, Viridi mentions various Piranha Plant subspecies, much to Palutena's dismay, so the latter decides she doesn't want to hear the name again, so Pit decides that his chance to get a tip from Palutena is gone.