The Phlask evolutionary line originally contained UFOs in their design.
The Diloweed, Phlask and Vectol lines were the last evolutionary lines to be confirmed.
The final design for Noxial was made by a different artist than the designs for Phlask and Fumighast. Both artists also created designs for the other Pokémon in this evolutionary line, which lost to the other artist's design(s).
The rejected Fumighast design (which went with the confirmed Noxial) had two heads, and required evolving both forms of Noxial together in order to evolve.
Phlask is based on a haunted chemistry set.
Phlask's name is a corruption of flask, a piece of glassware used in laboratory chemistry. Its name also invokes phantom due to its spelling.