It is a play on the Italian supercar manufacturers Lamborghini, Pagani and Italian motorcycle manufacturer Ducati. Only one of their cars are featured in GTA IV. In the downloadable content for GTA IV, The Lost and Damned, two of their bikes are also featured.
In GTA IV's audio files, this manufacturer is misspelled "Pegasi".
Perhaps coincidentally, there is a Spanish car manufacturer called Pegaso which briefly produced supercars during the 1950's.
This manufacturer's logo is similiar to the Lamborghini logo.
The name Pegassi is an indirect knockoff of the Ferrari horse emblem. Pegassi is believed to be derived from the word 'Pegasus', which is a greek mythology for a winged horse creature. The logo on the other hand features a horseshoe placed in between a pair of wings.
This manufacturer in theory would be owned by Vulcar, as both Lamborghini and Ducati belong to the Volkswagen group.