Count Giannino Marzotto thought Ferraris too heavy and too slow, so he made his own, using Ferrari chassis and powertrain and radical notions of improving aerodynamics.

he started with a Ferrari 212 Export Berlinetta engine mounted on a 166 Touring Barchetta. At least, that is what the translated text seems to say at http://www.elenaferrari.net/uovo.html

http://slotadictos.mforos.com/966558/11399961-la-mille-miglia-segun-carrera/
http://www.elenaferrari.net/uovo.html
http://propublica.com/UnZip/www/Mythos/6.html

This absolute one-off legend is the result of a collaboration between Carrozzeria Fontana and sculptor Franco Reggiani, which was commissioned by owner and racing driver Count Giannino Marzotto in an attempt to one-up Ferrari in the 1951 racing season. The objective was to create a more aerodynamic and streamlined body, and the result was the affectionately nicknamed ‘Uovo’, or egg.

While the design may have divided opinion in the pits, once on track it was clear that this 166 was a serious adversary, competing in events all over Italy and slowly proving its worth to Ferrari. After its retirement in the mid-1950s, the car was owned by a succession of owners until 1986, when it was returned to its mother country and hidden under lock and key, being seen only briefly while on display at the Museo Enzo Ferrari.

https://www.classicdriver.com/en/article/cars/unique-ferrari-uovo-has-been-cooped-20-years