After Jissonji is killed in the Nue arc, the monstrous, goopy form he adopts looks a lot like a kind of yokai called a “dorotabō.” Literally translated as “muddy rice field monk,” dorotabō are mud-covered ghosts that haunt neglected rice fields.
Jissonji post-transformation doesn’t have all the traits of a typical dorotabō. Dorotabō, for example, traditionally have three fingers on each hand and are said to be the restless ghosts of farmers who see their beloved fields neglected after their deaths. Jissonji still has all his fingers (I counted them) and isn’t really associated with rice fields or farming. However, Mononoke has kind of a lax attitude when it comes to depicting yokai. Most likely, Jissonji transformation is meant to be some kind of joke relating to his bald head. Many yokai that have nothing to do with the clergy are referred to as monks because of their bald-headed appearance, presumably reminiscent of a monk’s shaved head. The umi bozu (“sea monk”) is another example of this naming scheme.