I have just found out that Medusa Halloween costumes are a very popular choice, so let's see what can we do about that!
As regards her outfit, we don't know too much about it, because she is represented mainly after being beheaded, poor thing! So I guess a "peplos" or "chiton" would do just fine, or else a simple tunic with a decorative belt. Speaking about the belt, here is one image of Medusa where we see her entirely, and she has a belt made of intertwined snakes: Medusa on the Temple of Artemis. She is wearing a simple short tunic, with a kind of Greek key pattern.
For a better idea about how her outfit was represented in ancient statues, vases and more modern painting, visit my page with Medusa pictures and scroll down the page.
Like we said before, the secret for the succesfull Medusa Halloween costumes are the correct accessories. In Medusa's case, the particularity is obvious: her head full of snakes instead of hair. A simple way of doing this hair is to draw snakes on a piece of cardboad, cut out the profile and wear it like a mask, on your forehead, tied behind with an elastic. You could glue all kinds of sequins or cords to the snakes, so as to make them 3D. Make sure to comb the rest of your hair as untame as possible!
You could also use a simple plastic headband, matching the colour of your hair, and cover it with rubber snakes (see 12 Stretchable Toy Snakes
for instance). And, if you ask me, I think you could make a pony tail in which you add the above-said rubber snakes and then let everyone know that poor Medusa is having "a bad hair day".
Here are some headpieces and wigs that I found on Amazon.com (search for Medusa)
Let's not forget the make-up. You could either do something very scary, with lots of black, green and blue, or else make a nice makeup and just enhance the snake-hair. For tutorials and ideas about Medusa makeup, head over to Youtube.com.
You could also add some snake shaped accessories for your Medusa Halloween costumes (armband, necklace, earrings) - which can be recycled for a Cleopatra outfit, too! My favourite one is the last necklace, which also lights up and you can give it whatever shape you like!
Last, but not least, here are some images of Medusa Halloween costumes, as you can see they all are somewhat similar. Take a good look, maybe it helps for inspiration :-) The colours are usually black, green or dark blue (snake-like colours). My favourite is the short dress because... the skirt lights up!