Treant

The Treant is a Monster class in Disgaea: Hour of Darkness.

This class is immune to ailments from Tier 5 onwards.

In Disgaea 1 Complete they are replaced by the Shroom class, though the change is purely aesthetic and they function the exact same way.

Quotes
"Treants live throughout the Netherworld. Descendants of the Millenium Tree that grew of it's own will. They look like plants, but they are carnivorous and capture their prey by paralyzing it with poison gas. They can live for a million years, but depending on the region, they can easily die."

- Disgaea DS Collection Book