Easter Bunny
Explanation
The Joke
A woman is excitedly telling another person (Jenny) about the Easter Bunny, emphasizing the biological absurdity: "It's a mammal that lays eggs!" Jenny responds with a disinterested "So?" The woman then ups the ante, declaring with great enthusiasm: "It's a monotreme, Jenny. A MONOTREME." The bottom caption simply reads: "Cloaca."
The joke is that the woman is not excited about the Easter Bunny for the usual childlike reasons (chocolate, holiday fun) but because of its biological implications. If the Easter Bunny is a mammal that lays eggs, it would be classified as a monotreme -- the order of mammals that includes platypuses and echidnas. And monotremes have cloacas (a single opening for the digestive, urinary, and reproductive tracts), which is the final anatomical detail the comic leaves the reader to contemplate.
The Humor
The humor works by taking the familiar, whimsical figure of the Easter Bunny and subjecting it to rigorous biological classification. Instead of accepting the Easter Bunny as a magical holiday mascot, the woman treats its egg-laying ability as a serious zoological data point that leads to increasingly uncomfortable anatomical conclusions. The single-word caption "Cloaca" serves as a deadpan punchline -- the logical endpoint of this line of reasoning that nobody asked for and nobody wanted to think about.
References
Monotremes are the only mammals that lay eggs. The group includes the platypus and four species of echidna. A cloaca is the common cavity into which the intestinal, urinary, and reproductive tracts open in monotremes (as well as birds, reptiles, and amphibians).