The phrase "am on the horizon" refers to a state of appearing or emerging, such as a ship approaching the shore. Antonyms for this phrase include "disappear," "vanish," "recede," or "retreat," which suggest a movement away from the horizon. Other antonyms may include "hidden," "obscured," or "concealed," indicating that the object is not in view or obscured from sight. Additionally, antonyms for "am" may include words such as "was" or "will be," indicating past or future states rather than an immediate present state. Ultimately, the choice of antonym depends on the context in which the phrase is used.