That's A Plume Fossil
I Chose A Cover Fossil In The Relic Castle :(
I Wanted Archen... Well Now I Do,
And I do Not Want One From The
Guy In The Twist Mountain But
If That's The Only Way Than OK
I'll Get It From There :)
If you've chosen Cover fossil and saved the game, it's pretty much too late to get a Plume Fossil by yourself.