This page of the guide to The Witcher Monster Slayer contains detailed description of the second quest in the game - Winged Bandit.
Rewards: 1000 XP, 100 gold coins.
Right after completing the first quest - Final Exam - you will run into Thorstein. He will tell you about the monster that took his horse.
At the marked point on the map you will find a head of a horse. You must examine it closely to discover who killed it.
As you go further, you will come across a griffin. Defeating this monster is fairly easy. If you have trouble with it, though, invest in some oils and potions. Swift and Hybrid Oil will work well.
For defeating the griffin, you will receive a map that will lead you to a secret treasure.