According to Encyclopedia Mythica, "The Greek goddesses who presided over the arts and sciences. They were believed to inspire all artists, especially poets, philosophers, and musicians" (source). Generally speaking, a muse is any person who inspires a creative work. At the end of The Outsiders, before he begins to write, Pony tells us about his inspiration to pick up the pen:
Remembering a handsome dark boy with a reckless grin and a hot temper [Bob]. A tough […] boy with a cigarette in his mouth and a bitter grin on his hard face [Dally]. Remembering […] a quiet, defeated looking sixteen-year-old […] [Johnny]. One week had taken all three of them. And I decided I could tell people. (12.71)
Although Pony has very different feeling for all three boys and had very different experiences with each of them, they do come together to inspire him to tell his story, and in a rather creative way.