We found this book at our last visit to the library, and it was a big hit with Porthos (aged nearly 4). It’s part of the Dr Seuss Beginner Books series, and features Robert, a horse who is allergic to roses. His ailment trips him up time and again with hilarious consequences (and illustrations to boot), but he finally saves the day, and himself.
I lost count of how many times Porthos made me read it aloud, especially the “Ker-Choo!” bits. There is a fair bit of word repetition which is helpful for learning new words.