While this is the first time we’ll be seeing the Antithesis, there are other instances of beings who have acted as counter-existences to Galactus. In the early 2000’s, when Galactus was actually killed off, a being known as Abraxas appeared to cause havoc on a universal scale. Technically, he was described as the antithesis of Eternity, but it took the resurrection of Galactus to take him out of the picture.
During the Annihilation event, they introduced cosmic beings Tenebrous and Aegis as counterparts of Galactus who were corrupted in the universe’s youth and sealed away. They were mainly brought in for the sake of explaining how Annihilus had Galactus captured. “Two evil Galactuses beat up neutral Galactus,” is a pretty easy explanation.
Then there’s Galactus’ role in Earth X, a miniseries about an alternate dystopian future. In this continuity, it’s explained that Galactus’ great role in the universe is to keep Celestials in check by devouring their eggs (which, as it turns out, are hidden inside planets, such as Earth). Mutants and other enhanced humans are the Celestials creating white blood cells to stave off Galactus. Either way, it’s not good news for Earth.
In recent times, Galactus became his own antithesis when he was reborn as the Lifebringer. Instead of devouring worlds to absorb energy, he would go from planet to planet, releasing his own energy to make the worlds inhabitable. Coincidentally, this is an idea first introduced in the Spider-Girl MC2 continuity after Galactus and Silver Surfer merged into one being.