An engineer that is competent, good looking, dresses well and is not socially awkward.
John: How come no other engineers are like David?
Mary: He's an engineer unicorn. They are rare.
The legend says some of them can be found here http://www.engineerunicorns.com
"Unicorn" is a common term for an engineer (usually front-end) who also has good taste in design and is able to contribute to UX early on. But "unicorn" is also used for startups that turn out to be breakout successes, like Uber and Airbnb. Silicon Valley really likes unicorns.