I think that middle evolutions that are not starters are usually forgotten, because at least with fully-evolved Pokemon, you identify them as such. Things like Farfetch'd, Dunsparce and Stunfisk seems like they should be forgotten, but are still memorable in some way. For the base form, they have a certain level of cuteness to make them remembered.

Middle evolutions are forgotten because they are usually just within the evolution line between the form you start and the form you finish with. This is worse if that Pokemon is conceptualised as a middle form right from the beginning, because that means they are a "middle-child" from the beginning.