Umbrella Slime

Rain on the surface is the only thing that puts these outside, and while it lasts they turn up in place of the small slimes you are used to. That is a real step up rather than a recolor: bigger, tougher, and enough to end a fresh character who wandered out in a shirt with a copper shortsword. Royal Gel switches them off along with the rest of the slime family, which is the cleanest answer if you plan to keep building outdoors in a storm.

It never drops as an Umbrella. What you can get instead is the Umbrella Hat, a vanity piece, and that is the whole payoff. They are also the one slime in the game that can die without handing over any Gel at all, so a rainy day gel run is slightly less dependable than it looks on paper. Worth zooming in on the sprite while you are out there, because the previous owner's hand is still attached to the handle.