“Swatchlings are broad, black-feathered, bird-type monsters with a white mask which resembles that of a plague doctor's. The Swatchlings appear wearing a brightly-colored suit and tie with a white shirt underneath along with white pants and black dress shoes. As their name suggests, their suits, ties, and eyes may differ in terms of colors, from red, orange, green, yellow, and blue.”

- Description obtained from the Swatchling page on the Deltarune Wiki

Swatchlings are enemies (more commonly NPCs) currently encountered in Deltarune: Chapter 2.
Their name is a pun on “Swatch”, a sample or book of colors to use as a reference, and “Underling” as they do appear to be subordinate to Swatch.