Farmworld Simon Petrikov is Simon Petrikov from the Farmworld timeline. He froze the bomb that would have brought the Lich into existence in mid-flight, and perished underneath its massive weight when it landed on him. Farmworld Marceline guards his body. This eventuality exists due to Finn's wish of the Lich never existing during the episode, "Finn the Human." Another result of Finn's wish is that the Ice King never existed because Simon had never fully transformed into him. Even though he is dead, Marceline imagines his voice and he orders Marceline to retrieve the crown after Farmworld Finn stole it.
Farmworld Simon looks exactly like a mutated Ooo Simon. He has a beard, which is not as long as the current Ice King's beard and wears a tattered suit. He is also always seen wearing the Ice King's crown.