Here's a sliver of resoning I haven't seen yet.
Mantine and Skarmory were seen as parallels because they have the same BST and similarily distributed stats with one being oriented more for physical purposes and the other for specials.
Gligar and Delibird were also seen as parallels, mostly because they were version exclusive parallels. They're stats weren't even (Gligar's BST is 100 points more than Delibird), but you still found them at roughly the same points in time in the game.
Originally, Skarmory and Delibird were only available in Silver.
Mantine and Gligar, the version exclusives of Gold, were both given an addition to their evolutionary lines. If Game Freak really did want to use these pokemon respectively as parallels, I'd predict a skarmory prevo and a Delibird evo. ToddlerSkarmory would have the stats 45/60/120/60/20/50, and SuperDelibird would have the stats 70/75/70/100/70/95.
Serving up shade since it was relevant. Plus, TEAM CHESPIN
"Speculating about it isn't going to give us any answers" -Virizion, PMD Magnagate
Shaddup, Virizion. You don't know nuthin'.