Mostly Electivire, Magmortar and Rhyperior. They evolve with trade items, (not Tangrowth) all are fully evolved, (most have 2 evolutions) and all have either 535 or 540 base stats. If not Tangrowth, is he connected with Yanmega? (both having 1 evolution by learning Ancient Power)
Electivire, Magmortar are counterparts but alot of people like to speculate that Jynx is a counterpart to them too. There was a rumor going around years ago that Magmar, Electabuzz and Jynx were orginially gonna be instead of the Bird Trio in Red, Blue and Yellow. As for Rhyperior and Tangrowth, they have no relation really.
Hope I helped!
Rhyperior, Electrivire and Magmortar all need special items and trade to evolve (Protector, Motor Drive and something else). But Tangrowth isn't really related.