Rare Beasts is the first book in the Edgar and Ellen book series.
The cover artwork shows Edgar to the left, and Ellen to the right (the same can be seen in the picture below), while Pet, Edgar and Ellen's pet, can be seen in the upper-center of the book. In the background, the twin's home can be seen.
From The Back of The Book
Fire ants. Fraud. Footie pajamas.
Twins Edgar and Ellen live alone - their parents disappeared years ago, and who can blame them? - in the quaint, little town of Nod's Limbs, in a grim, gray house overlooking the cemetery and the junkyard. They spend their days avoiding Heimertz, the mysterious accordion-playing caretaker; pestering Pet, a hairy, one-eyed creature of indeterminate species and gender; and wreaking havoc on the hapless citizens of Nod's Limbs.
But wreaking havoc can incur expenses, so the twins come up with a unique fund-raising scheme: They'll nab the pets of Nod's Limbs and transform them into exotic animals they can sell for big bucks. Not a bad plan, if one of the purloined pets wasn't a lethargic python with a raging appetite....
The next day, the twins decide to play a game of modified version of "Hide and Seek" (in which you must not only find the other participants, you must also subdue them), but when Ellen is about to slice her brother open with a pendulum after finding him, the twins decide that they are bored with their normal games and decide to torture Pet.
Pet is found watch an episode of Professor Paul, and episode on "Exotic Animals". This gives the twins an idea: If they could find their own exotic animals to sell, they could make money to finance their future pranks.
The twins then proceed to kidnap all of the town’s pets and alter them with glue, Christmas decorations, paint, markers, and glitter, and to then re-sell them as "Rare Beasts".
Edgar and Ellen go into town to sell their ersarz exotic pets. The twins attept to sell their pets to Marvin Matterhorn, a milk woman, a mail man, and a zoologist. Although they don't succeed due to their high prices, and due to the fact that nobody seems to like them due to their bizarre aspect.
The twins, furious, blame each other and start fighting until the children realize that the so called "exotic animals" are actually their missing pets. After retrieving their pets, the children attack the twins and douse them in water. Meanwhile, Van Barlow's jar of fire ants brakes and the ants bite Edgar and Ellen.
When everything is over, Edgar and Ellen return home and go to bed as Pet finishes watching Professor Paul, who then reveals to the audience which is the rarest animal on the planet: an animal that, in description, greatly resembles Pet.