Lucario is an anthropomorphic Pokemon that stands on it's hind legs (though I'm sure it could be on all fours if it wanted). Lucario can use a special ability to manipulate Aura--it's it's natural ability. Lucario is the 1st Stage Pokemon to Riolu, a smaller, cuter Pokemon. Riolu was revealed after Lucario. Lucario is also the star of it's own movie, despite not being legendary.
Zoroark is an anthropomorphic Pokemon that can stand on it's hind legs. Zoroark can use a special ability to create illusions--it's (likely) it's natural ability. Zoroark is the 1st Stage Pokemon to Zorua, a smaller, cuter Pokemon. While the two were revealed at the same time, Zoroark was technically revealed first, through the silhouette. Zoroark is also the star of it's own movie, despite not being legendary.
Then, look at the physical aspect:
Both Lucario and Zoroark have three digits.
Both Lucario and Zoroark have small upper arms, but larger forearms.
Both have something coming from their heads that trails behind them. Lucario has appendages; Zororark has a mane-like extension.
Both Lucario and Zoroark have long, pointed ears.
Both Lucario and Zoroark have body parts that closely resemble man-made objects. Lucario has metal on it's chest and paws; Zoroark has a ring in it's mane.
Lucario is Blue and Black; Zoroark is Red and Black. (Red and Blue are often seen as counterpart-colors.)
See the resemblance? The two Pokemon are very similar.
However, Zoroark is definitely not some sort of byproduct of Lucario or anything--it definitely has enough differences. I believe they could be counterparts, but I think there could be a better word for this pair.
They two are parallels to one another, I think. Since Aura and Illusions may or may not go together in the Pokemon world, you can't really say for sure yet if the two are counterparts (for all we know, Aura could be the source of the Illusions--we don't know yet). They definitely parallel one another, however, for reasons I listed above.