If you've spent time on the internet, you've probably heard of Snake Island, Rabbit Island, and Cat Island, all great examples of how the size and isolation of islands can occasionally cause some wacky/horrifying ecosystems to develop. (It helps if said island was home to a secret chemical weapons factory, rendering it unfit for human habitation.)
The map above shows islands/isles/cays with an extreme or otherwise notable concentration of specific animals. In many cases these populations are seasonal (like the Christmas Island crabs) and also, with the exceptions of Ball's Pyramid and the Galápagos, these aren't the only places densely populated by the listed animals. I had a couple other options to choose from for penguins, monkeys, and bears, and I almost went with Ap Lei Chau in Hong Kong harbor—which has a slightly lower density but much larger population than Santa Cruz del Islote—for "people island".
See anything I missed? Anyone know if there's a spider island anywhere? Leave it in the comments.
h/t to the excellent Atlas Obscura, which got to this well before I did.