Zombie: mental changes could include a horde mentality (a preference for being in a pack of people), a voracious appetite, or slow mental processing speed. Physical changes could include rotting flesh, jerky walking movements, detachable limbs, or the like.

Merperson: mental changes could include a newfound obsessive love for music along the lines of sirens, a desire to lead people to watery areas, and secrecy. Physical changes could include scales or gills, a tail, or shark teeth.

Harpy: mental changes could include loving shiny objects, a need to nest, and a territorial nature. Physical changes could include wings, talons, or a beak.

Ghost: mental changes could include being very attached to one location, object, or person (i.e. haunting), a penchant for holding grudges even if the character in question is usually very forgiving, and an obsession with the past. Physical changes could include intangibility, becoming see-through, or even becoming a generic Halloween sheet ghost if you so desire! Get creative with it!

Golem: A quick note on what we mean by "golem" - it's basically a creature made out of one sort of object or substance, like a brick golem or clay golem or butter golem. You can pick absolutely anything for your golem self to be made from. So, physical changes could range a wide variety of places based on what you chose! A flower golem, for example, could have thorns for nails and teeth and vines for hair; a brick golem could have caulk flakes instead of dandruff. Mental changes could have to do with what they're made out of - for example, a flower golem might have more vibrant and "growing" sorts of emotions, being much more flexible than a brick golem who would become a much more inflexible sort of person.

Slime Monster: mental changes could include a slippery moral mentality, being "slippery" or cagey in regards to their feelings, and feeling a need to consume and/or absorb something in particular or everything in a katamari sort of way. Physical changes could include the ability to become a liquid or to shoot out sticky goo at people. Helpful for Halloween pranks!

Vampire: mental changes could include being obsessed with numbers and counting, doing things very correctly, a flair for the Dramatic, and having to be invited into places. Physical changes could include fangs, changing into a bat, or sparkling like Twilight vampires if you really want.

Mummy: mental changes could include suddenly holding onto grudges (even for things long forgotten), hunting people down for revenge or retribution, or a desire to rule over things. Physical changes could include extra appendage mummy bandages or a gaunt demeanor.

Shade: mental changes could include wanting to disappear into the shadows, being secretive, or a desire to consume people's shadows. Physical changes could include being made of shadow, having a large shadow or a shadow that acts independently of you, or creepy shadowy eyes.

Werewolf: mental changes could include being more tempermental, have more puppylike instincts, or being more loyal. Physical changes could include changing nightly instead of only at full moon, being hairier during the daytime, or having sharper teeth.

Demon: mental changes could include liking making deals, a lack of understanding morality, and extreme greediness. Physical changes could include horns, talons, or wings.

Evil robot: mental changes could include becoming very logical and systematic, a lack of imagination, and a very white and black no shades of gray morality system. Physical changes could include wires for veins, metal skin, and requiring batteries or charging to live.