You can do a smart pet search to say Pet Battle and Has Conditional Spawn to get a list of the 12 pets that have some requirement for them to spawn. Those are all probably going to be harder to catch a rare than a normal pet but how much so kinda depends on the exact condition. That is a good start but it doesn't cover all the "hard to capture" wild pets. The luyu moth was a challenge for me too because there seems to be a relatively limited number of spawn points for it. The Alpine Foxling Kit is similar. I've managed to get my rare Luyu moth but not my rare Foxling kit. Of course someone can always get lucky and get either of them on their first battle.