A derogatory (slang) term to describe anything other than a Spellbinder, or Dragon.
Analysis EditActually, the term has on and off has been used to describe, werewolves and Vampyres, elves, sometimes dragons (Master Chem has said it before) and mermaids.
Almost every villain in the series (except Henri) has been a Semchanch and, oddly only two Semchanchs have reappeared in later episodes, for example, Selenba. In which is most more similar to the Monster of the week example, where every episode there is a new creature.
All semchanchs have a aura around them. Vampires and Werewolves have an amber/scarlet, dragons have a gold and elves have a green. Mermaids gain a bright blue aura when they are underwater and a few harpies have a white aura when in flight.