The creature on the front of the book is Phyl, a Platypus. A platypus is a strange creature from Australia that is a Mammal, and yet will lay eggs. For this reason, it is a good illustration of two important ideas fundamental to object-oriented programming, namely inheritance and overriding. You can learn more about these concepts, and about Phyl, in Chapter 1.
Here is a picture of phyl and me. You can find more pictures of phyl, as well as some funny stories concerning us both, at my ftp site ftp.cs.orst.edu/pub/budd/oopintro/2ndEdition.