Invisible Eagle

As the name implies, this creature from the planet Neptune was a giant, invisible eagle, slightly smaller than human size.

In the first Legion story in Adventure Comics #247 Superboy failed his Legion initiation test -- to rescue the Nova Express -- when he was distracted en route by calls for help capturing the Invisible Eagle, which had escaped through a gaping hole in the roof of the Interplanetary Zoo and therefore might "bump into" rocket-liners.

In the time it took Superboy to find the eagle -- he snatched a sea-borne iceberg and flew around 30th century Smallville until the drop in air temperature caused frost to form on the eagle's wings -- Lightning Boy was able to reach the Nova Express first.