News at Eleven (Back Page): [Ernest Lawrence] Thayer always insisted,

despite some ballplayers' claims to the contrary, that Casey was not based on any specific ballplayer, since he didn't follow baseball that closely. As for why he chose the moniker "Casey," he just liked the gritty sound of the name from the time he first heard it on the schoolyard playground in that skirmish with classmate Danny, Danny Casey.

from The Washington Times: The mystery of "Casey At the Bat" as he turns 125

~~~~~~~~~~~