Pfister is based on Porsche. The Pfister logo is in the shape of a shield, just like
Porsche's iconic logo. The name is a reference to Pfizer, a brand who produce the viagra medics in 1999. [Supposition - Consider revision.]
Pfister is probably owned by BF, similar to how Porsche is owned by Volkswagen.
"Pfister" is likely another piece of adult humour by Rockstar, meaning "fister" as in someone who sticks their fist into the female genitalia or male or female anus.
In GTA San Andreas, pedestrians in areas like Paradiso and San Fierro can be heard saying what sounds like, "Have you driven a Pfister?" Sometimes, if the player is driving an expensive-looking vehicle, a pedestrian might comment positively, "Is that a Pfister?" This could mean the brand was planned to be introduced before GTA IV with the scrapped system of vehicle brands (as seen in carcols.dat).