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%)||Soul of Thamasa|
|Uncommon - (10%)||(Nothing)|
|Rare - (3%)||(Nothing)|
* Ishteen’s chances of appearing in either of these two zones of the Barheim Passage increase with time. Every ten seconds there is a 2% increased chance that Ishteen will spawn close to the party.
Note that you need to have initiated the Hunt Club side quest in order for Ishteen to spawn.