You'll find a great history of the Eagle award in the Scouting magazine archives.

Robert S. S. Baden-Powell's introduced badges for achievement in his 1908 book Scouting For Boys. It is said that Baden-Powell got the idea of awards from the American naturalist Ernest Thompson Seton, who passed along ideas from his Red Indian Boy Scouts of America. Initially, the highest available rank in Scouting in the U.S. was going to be the Wolf Badge, based on the Silver Wolf badge in Great Britain.

However, no Wolf badge was ever awarded.  Several leaders who reviewed the proof version of the Handbook for Boys of 1911 thought that the highest award should recognize the American Eagle.  So by the time the handbook was published, the highest award in Scouting in the United States was given its current name - Eagle.

Initially, the Eagle award was described as the "highest scout merit badge".