I keep getting Zombie Villagers at night - Minecraft PC 1.12.1
Zombie villagers are a hostile mob that make up 5% of zombies that spawn in the Overworld. They will spawn just like normal zombies. You can capture and heal them to get villagers at your house if you would like.
Quote from the wiki:
Zombie villagers also spawn naturally in the Overworld in the same conditions as a normal zombie.