We can intuit if that's the case based on the internal list actually. While Pokemon constantly were changing designs (sometimes to the point of being replaced with another Pokemon in the line because apparently moving sprites was easier than changing stats), Pokemon that have a mechanical reason for existing seem to be designed around the same time (Snorlax and Mewtwo for story reasons, starters mostly next to each other as they finalized the concept, filler version exclusive grass types at the very end) so that's a reasonable indication. Unfortunately we have a problem there because Golem and Graveller were both designed fairly near each other and seemed to be unrelated, and the Machamp line is all over the place, but Machamp itself is later than every other part of the line. In addition, Haunter, Abra, and Alakazam were designed around the same time, which says to me they were filling out those lines with the intention of using them as trade evolutions. For the record, Gengar and Gastly have the same "designed really early and might have been unrelated" going on. Based on the evidence I've seen, your statement seems inconclusive at best.
