From The Aquarium Wiki
The following plecos species can be labelled as Common Pleco.
Due to the number of species in the genus, identification is very difficult without exact provenience of the specimen. Most common plecos are now exported from Peru and Brazil. Photos of the real H. plecostomus are common in aquarium literature from the 1960s and 70s when common plecos were exported from Venezuela, Suriname, and the Guyanas. Most plecos labelled as 'common' come from the Hypostomus and Pterygoplichthys families and will grow exceptionally large needing a tank of at least 100 US gallons.