Blue Slime

Daylight on the surface is the safe window. Out in a forest during the day it bounces around and ignores you until you hit it first. Underground it is hostile on sight at any hour, and after sunset the surface ones switch over as well. That flip is the first difficulty step a new world hands you.

• Forests during the day and the underground layer at any time, through ordinary spawning. Nothing summons them.
• Slime Rain drops them out of the sky regardless of biome or hour.
• On a windy day some arrive hanging from a Windy Balloon, dropping in from above.
• King Slime spits them out whenever it takes damage, so its arena fills with them for free.
• A Slime Statue makes them on demand, but statue slimes never carry the holiday bonus drops, so that farm is worthless for Presents and Goodie Bags.
• Normal worlds block them from spawning in a wide band around the world spawn point. Expert removes that buffer, so a fresh Expert base has no quiet zone around it.

Royal Gel makes the whole slime family walk past you like scenery. During Slime Rain that turns an event into background noise.