The Mad Monks, alternatively known as Grounds Keepers, are the inhabitants of the Asylum Grounds which they wander with shovels in their hands.
Behind the scenes
In their initial concept art and the MediEvil Rolling Demo, the monks sport a human appearance, instead of an undead one. This was likely altered to avoid religious backlash. Their design was further altered for the US release of the game where their costumes were retextured to look more like that of a gardener than a monk. According to developer Jason Wilson, this change was made due to cultural differences, as some areas of the US were thought to be deeply religious.