My daughters are both thoroughly obsessed with mermaids. So, I bought two of these (exactly the same, so no fighting) from Target.
The tail on the Barbie is made of a kind of squishy rubber type material. And, if you push on the sides of her hips, she flips her tail up and down. My only complaint about this feature is that smaller children may not be able to do this. My four year old has fairly small hands, and she's rather upset that she can't push both sides of the hips and make the tail flip as well as mom can.
Each of these (there are 4 different types, in three different colors) come with the tops painted on, and the tails firmly attached. She doesn't have legs, so you can't really dress her in other Barbie fashions. They each come with a sea friend (the one I bought came with a small dophin, the pink one comes with a yellow fish, and the blue one comes with a pink seahorse).
My kids absolutely love these things! The little ocean buddies that come with the doll can attach to the tail with a small ring that comes with. But, my kids prefer to play with the friends seperately. They've already taken these into the bath, and they had a blast there too!
This is a really great gift idea, in my opinion. And, these aren't terribly expensive, while still having the cool features of a) being a mermaid, and b) having a tail that can flip about making her seem more life like. And, the tails are glittery.