This is some fan-art of Eustace as a kangaroo monster from the episode, The Transplant. In this image, Eustace and Muriel were given alternate color palettes. Eustace is a green kangaroo monster instead of a red one; Muriel has orange brown hair, and she's kind of dressed like a clown, as she sits in Eustace's pouch. In his Kangaroo form, Eustace is standing on a rock with Muriel in his pouch as the two of them look at the sun, which is smaller than Eustace now that he's a kangaroo monster.
(References: The Transplant; Courage the Cowardly Dog; Muriel; Eustace)