I suspect this won't constitute the droids you are looking for, but after reading up on the subject it's the best I can offer at this time.
Werewolves spend most of their time as humans (whether wizard or Muggle). Once a month, however, they transform into savage, four-legged beasts of murderous intent and no human
Fantastic Beasts and Where to Find Them - Page x - Scholastic (US) Edition
I can't give exact percentages, for the precise number of werewolves is likely known only to the Werewolf Support Services at the Being Division and the Werewolf Registry and Werewolf Capture Unit of the Beast Division of the Department for the Regulation and
Control of Magical Creatures. Even then, I would suspect that not all werewolves register -- can you see Fenrir Greyback complying with any Ministry decree? I can't, at least not readily.
Several highly intelligent creatures are classified as “beasts” because they are incapable of overcoming their own brutal natures.
FBAWTFT - Page xiii - Scholastic (US) Edition
Humans turn into werewolves only when bitten. [SNIP] Once a month, at the full moon, the otherwise sane and normal wizard or Muggle afflicted transforms into a murderous beast.
FBAWTFT - Page 42 - Scholastic (US) Edition
Note: I interpret the above to mean that the werewolf bite is the usual means of passing on lycanthropy, but canon clearly states that it can be passed along genetically as well. Lupin notes that 'his kind' rarely breed, but it is possible for a werewolf to do so.
So in Potterverse, lycanthropy can be either brought upon a wizard or Muggle through a werewolf's bite or it can be passed on genetically. Voldemort references the genetic component in the chapter The Dark Lord Ascending in Deathly Hallows when he asks Draco if Draco will be "babysitting the cubs" in reference to Lupin's and Tonks's offspring. Lupin himself bemoans the fact that he could have possibly passed on his lycanthropy to his unborn child in the chapter The Bribe in Deathly Hallows.
So, given the two means of transmitting lycanthropy -- through bite or genetics -- within the population of werewolves, there will be a percentage of werewolves who will be incapable of magic under any circumstances -- Muggle werewolves for certain, but perhaps also the offspring from a Muggle and wizarding werewolves, depending on whether the wizarding gene manifests. Muggle and wizard werewolves are classified as werewolves without distinction to their origin (wizard or Muggle) and are overseen by the Department for the Regulation and Control of Magical Creatures (how a Muggle werewolf would know how to register I can't say.) If a half-blooded wizard werewolf bred with a Muggle werewolf and produced werewolf offspring, there is a chance the offspring might not be magical. Conversely, perhaps it would be possible for two Muggle werewolves to produce a wizard werewolf, just as Muggleborn witches and wizards occur. So here is a genus of magical creature that would presumably always have a percentage of non-magical specimins.
Disclaimer: I'm no rocket surgeon, so I'm coming from a layman's POV.