Well...someone finally did it. A human tail has been invented that wags when the wearer is in a good mood. The 'Tailly' is attached to a belt with sensors which takes the wearer's pulse. When the heart beats faster, the tail wags faster (you may not want to wear it while excercising or meeting very attractive people because of that, although that would be a really easy way to see if someone likes you. Sorry. Tangent). It was designed to be fun for parties, while playing with your kids (but perhaps weird if you are playing with the neighbor kids), and maybe while you are out having adult beverages with friends.
Just please be careful you don't get so into character you find yourself lifting your leg.