Anti Starco is the paring of Star and Marco as rivals or enemies.
Canon Probability Edit
Although Star and Marco have opposed each other many times, the pair becoming enemies or rivals is highly unlikely considering their string bond of friendship. That does not mean that the "ship" will NEVER HAPPEN. Rather, think about the characters in Pokemon, the example being Red and Blue. They are friends, and yet they still battle each other. Think about how rivals push each other to get better. That is the most likely what Anti Starch really is.
Daron Nefcy (Aka the show creator) originally was going have Star and Marco be enemies, and both being in elementary school.