The Rigellian Shrub is a limited time decoration released during the Treehouse of Horror XXV Event. It increases the Tree-hugging rating on the Conform-O-Meter. There was also a small chance that it could be found in a Treat Bag. The in-game description referred to it as a carnivorous plant.
It was re-released as a premium item for the Deep Space Homer 2016 Event.
It returned during The Invasion Before Christmas 2017 Event as a premium item bought in the shop.