This is a mini-quest which will not show up in the journal.
Just outside the NE gate of Oxenfurt there is a merchant advertising a "basilisk" to a small group of Redanian soldiers. When Geralt approaches the merchant, there will be some dialogue where Geralt reveals the merchant is lying and the "basilisk" in question is really a young, half-starved wyvern. After the Redanian soldier asks for a refund from the cheat, the cage door breaks open and Geralt will need to kill the freed level 10 wyvern. Once it is killed, there is a small XP reward.The easiest way to get to this quest without having to make a special trip to the area is during the main quest when Vernon Roche asks Geralt to meet him at the bridge into Oxenfurt. Fast travel to the Herbalist's Hut (closest waypoint I had) and then call Roach. Following the marker to Roche's location means Geralt will pass by the "basilisk" just before he reaches Roach's location.