There are no medusas in The Sims 2 unless your referring to a
console version, but on PC, you can be :
A Zombie, (req. University)
A Vampire (req. Night Life)
Build a Servo (sim can't become one, but can build one, req.
Open for Business)
A Zombie (req. Pets)
A Plantsim (req. Seasons)
Befriend a Bigfoot who laters moves in (req. Bon Voyage)
And a Witch (req. Apartment Life)
With FreeTime you get a Genie but he's not necessairly a
creature that you can play)