Windfang and Mutsutakashi are correct. The Thief Wolfen is suppose to be a super rare monster.
In fact in the game, it is suppose to be the designated random spawn monster in all areas of the game with a spawn rate of 0.1%.
Len's Thief Wolfen just so happens to be the one who has direct ties with it in the story due to what it did to his father.
I get your point about it being rare and I agree, but 0.1% isn't that bad. That's a 1/1000 spawn chance, and if we add a arbitrary number of 30s between encounters and 90s per encounter, that's 120,000s per Thief Wolven, or 2,000 minutes, or 33.33 hours on average.
By the game's logic, it's rare but it's not impossibly rare. You could easily hunt multiple per week if you're dedicated.
Edit: seeing 3 strike reactions, I assumed my math was wrong somewhere but it's not. 120s per encounter, 1000 encounters per Wolven (on average) = 120,000s, divide by 60 to convert to minutes = 2,000 minutes, divide by 60 to convert to hours = 33.33 (repeating of course) hours.
So I can only assume that people think a 1/1000 spawn rate in every map across an entire game is actually "super ultra impossible rare!" meanwhile, Shiny Odds in Pokémon are 1/4096 (0.02%) and used to be 1/8192 (0.01%). The game world's logic makes this Wolven actually not that rare.