Great question, the history has many twist and turns.
The name was first suggested in the 1920's by students, it replaced the animal that an early Can't State president raised on his farm...Silver Foxes. The Flash was represented by many things like a Golden Retriever, a Thor-like man with lightening bolts in his hands riding a noble steed.
Then in 1985, the Golden Eagle flew in for the rescue.
Boston College athletics, at the time, were called the "Golden Eagles" and instead of a name change K.e.nt decided to retain the original name with an appropriate mascot.
Since that time, the golden eagle has been the official bird mascot of the "Golden Flashes"