These are unique Witcher 3 monsters that are not contracts, but are related to quests and haven’t been mentioned in either our Boss Fights Guide, our general Monster Guide, or our Monster Contracts Guide.
The alternative, peaceful way of doing the quest “Contract: Deadly Delights”, against the Succubus. Yes, you don’t get the mutagen, but do you really need it?
In the quest Deadly Delights you uncover a Succubus living in the city of Novigrad. You can either warn it off, or fight it. This is the fight, but it’s clearly not meant to be fought.
Monster Contracts are side quests in The Witcher 3 that you can take on for extra money and experience. You usually get Monster Contracts from notice boards.