The Pegaso Z-403 Monocasco was a two-level monocoque (chassisless) coach, fitted with a 125 hp (93 kW) diesel engine asymmetrically mounted amidships, designed in 1949 and built in Spain by Enasa between 1951 and 1957.

The Z-403 featured a single chassisless structure which allowed a better use of space, with all the mechanical units located in the underside of the vehicle, isolated from the passenger compartment.

Its concept answered to the creation of a vehicle with a total length of 10 metres (32 ft 10 in), capable of transporting 30 to 45 seated persons depending on the comfort desired, and with good visibility.

The high resistance to flexion and torsion of the single-structure was ensured by the elements which made up the monocoque: the exterior covering of light, easily changeable metal panels.

The Pegaso Diesel engine with 125 hp (93 kW) was mounted in the central part of the vehicle, in the space under the half top deck, providing good weight distribution and high stability.