Cartesian equation:
y2(a2 - x2) = (x2 + 2ay - a2)2
The particular bicorn given by Sylvester and Cayley is a different quartic from the one given here but this one, with a simpler formula, has essentially the same shape.
The URL of this page is:
http://www-history.mcs.st-andrews.ac.uk/Curves/Bicorn.html