I'm suprised they never used Lions, Giraffes, Zebras, Bald Eagle, Ospreys, Whales, and Hyenas on the covers. Also, that they used starfish and anteater before using Elephant on the covers. I know this isn't about the story, but still these are animals I thought would be easy ones to put the covers.
Infact, I'm surprised they never used Lions (except for David), Giraffes, Zebras, Pandas, Hyenas, and Panthers in the series at all. They also seem like easy animals to use.