In the Witcher Universe "monsters" come from a variety of sources.
Some - as you found in The Witcher: Nightmare of the Wolf, are the result of deliberate or accidental magical creation. Many others, such as vampires, Griffins, and others were brought from other worlds during the Conjunction of Spheres (an event that occurred some thousand years previously that temporarily joined many disparate worlds).
However, the origins of many creatures are left somewhat vague, with it being unclear whether they are "natural" to the world that of the characters we see live in, or were brought in from one of the other sources I mentioned.
I can't recall, nor find any references in sources to any "monsters" which are explicitly stated by a reliable source to have existed in this world before the Conjunction - though there are some where it seems implied, such as with Sylvans, Spriggans, and Fiends