Every villager has a percentage chance and none are rarer than others. The only rarity is based on species types. The game has a complicated way of choosing villagers so I suggest that you check out the following videos on @Koramora 's YouTube channel. She goes very into detail and helps you understand the process! Everything is based on RNG or Random Number Generator! This website has alot of guides on New Horizons as well! It is a tab bit outdates though...
Not necessarily. Some villagers I got are fairly common but adorable. As Keroppi noted, rarity depends more on the species of animal you're looking for.