While I don't offer themes in my programs,* I will ask parents and caregivers what sorts of things their birthday children like to incorporate into a storytime performance. Recently, a parent told me that her young child liked all animals, but hippopotamuses, giraffes and monkeys were particular favorites. I composed a song using the "piggyback" method of taking an existing tune and creating new words for it. The song is called "The Hippo Swam Up the River" and it's to the tune of "The Bear Went Over the Mountain." I haven't tried it out yet, but you're welcome to give it a go in your storytimes or home surroundings. If you improve upon it, let me know. For a fun, original song in both verse and melody, check out Eric Ode's The Hippopotamus Song (complete with hand-motions) off of his "I Love My Shoes" album.
*In my experience, whenever I've had storytimes with specific themes, I've felt I often had to sacrifice quality for stories and songs added as padding to flesh out a set. However, just because I don't do themes doesn't mean you can't! Favorite hippo stories: everything about George and Martha by James Marshall, Hot Hippo by Mwenye Hadithi and Adrienne Kennaway, Hippos Go Berserk! by Sandra Boynton, and many stories in which these river-horses play supporting roles but secretly steal the show(s). Then again, I think of Little Gorilla by Ruth Bornstein as an ensemble piece.
Here's the song (click on the image to enlarge it):