The female version of a manicorn.

A woman who is:
attractive, but not arrogant,
well maintained, but not high maintenance,
inexpensive, but not cheap,
passionate, but not slutty
sensitive, but not easily offended
more logical than emotional, but not uncaring
attentive, but not smothering
faithful, but not boring
etc... etc... etc...

Basically, a womanicorn is the perfect woman;
a myth that keeps some men looking for "the one".