Bestiary entry from the Clan Primer:
This creature always drops a trophy when it is defeated. This trophy is used in the Hunt Club side quest which involves trading in the trophies for items, armor and accessories.
|Common - (55%)||(Nothing)|
|Uncommon - (10%)||Hi-Potion|
|Rare - (3%)||Scarletite|
The Aspidochelon has a 40% chance of appearing each time you enter the Feddik River area of the Cerobi Steppe. You can steal Scarletites from Aspidochelon so you should take the opportunity to steal as many of them as you can prior to defeating it and obtaining the Adamantine Trophy.