First of all, I want to say I am aware how incredibly unlikely such a feature would be, especially because of how much art and interactions would need to be changed and how the feature would be somewhat lore unfriendly, but I ask that you consider what this feature could bring to the table.
I'm not sure what kind of system would work best for causing such a transformation, maybe there could be a sort of experimental "lycanthropy" strain of infectious nanites that transform their victim between a human and werewolf form depending on whether its day or night. The nanite "curse" has some sort of lore reason for being able to somewhat effect the player (maybe its a particularly advanced strain of nanites, maybe its enchanted by demons who want revenge on the player so its an actually literally a curse).
There would be two stages and two ways to cure this condition,
Stage 1: You shift between forms like normal, most of the time you're human but at night you're an anthro wolf. This can be cured fairly easily with strange, rare flowers that bloom only at night.
Stage 2: After being infected by Stage 1 for a significant amount of time (about, 5 weeks or so, but the countdown is hastened significantly whenever you climax in furry form) you progress to Stage 2 where your transformation is now permanent, curing this is far more difficult and requires doing a special quest line.
Furry transformation mechanics:
-Your character now appears as a wolf anthro of generally the same body proportions.
-Characters may react to your transformation, some might REALLY love it.
-You can see slightly better in the dark.
-You have +1 to all your basic stats (str, agi, libido, etc.)
-Unarmed melee attacks are slightly more powerful.
-You develop furry fetish faster.
-If you climax inside an enemy, you'll knot which will make struggling much much much harder for a couple turns.
-Your enhanced sense of smell makes you much weaker to aphrodisiacs.
-Some enemies might try to leash you, making it impossible to run away from combat until you break off the leash, enemies can also pull you towards them with the leash. (Oh also there could be a Bondage XP thing for stuff like the leash, ziptie effect, tongue frog effect, etc.)
-Enemies might headpat you in combat, you can develop a weakness to headpats mwahahahahaha.