I have recently been thinking about Pokemon "required" for a generation to be complete. Obviously, a generation must have three starter families. But each generation seems to have other requirements as well, for example a weak mammal family in the early routes (Ratatta, Zigzagoon, etc.), a family of birds early on (Pidgey, Starly), and a family of metamorphic bugs (Caterpie, Venipede). Even in the legendaries this trend can be seen. Trios like the legendary birds or beasts appear, and every generation has a small fairy-like (for lack of a better term) legendary, such as Mew, Celebi, or Jirachi.
So my question to everyone is this: What other trends have you noticed that persist through the generations?
*As a note, Generation II seems to follow these trends to a lesser degree because of its close ties to Generation I. Trends don't have to be present in every generation, but if it appears in three or more, it probably is one.
Other than Gen 2, Ever Generation has a Fossil duo. I have a list somewhere of what is needed for each generation as I had started a Fakedex contest on one of my sites. Fossil and Pikachu Clone I think are pretty much the only things you missed though.