Piranha Plant becomes the new door guard for Peach's Castle when?
Bowser plants the plant near the castle door where he is keeping her.
Bowser: at all cost you must keep the princess inside the castle at al times! Got it!?!
The piranha plant nods and stands guard.
Later that day princess peach was walking around the empty castle and sees the plant guarding the front door through a window. She walks out and gets the plant's attention. He growls at her to see if he can scare her back inside.
Peach: Hello little one. I saw a nice flower bed over there and I wanted to pick some up.
The plant shook his head at the princess. He pointed at the door with his leaf.
Peach: Well if I can't go can you go get me some?
The piranha plant pondered and nodded as he came back with a mouthfull of vibrant color flowers. Peach smiles and goes to pet the plant but he flinches.
Peach: oh my... I'm not going to hurt you.
The plant extender his neck and rubbed its head against her soft hand. After that the princess goes back inside for a couple of minutes. She comes back out and walks over to him.
Peach: I have a gift for you.
She places a vibrant flower crown on him and smiles. He begins to weep and snivell as he has never been given a gift before.
Peach: how about we go to my home and you'll be planted in the royal garden where I'll visit you every day?
Piranha plant nods and nuzzles the princess. Later that day the piranha plant smiles as he wears his new flower crown as he dances in the royal garden.