Kevin is a giant bird of the species that explorer, Charles F. Muntz has been searching for for many years.
Russell, who finds the bird, immediately decides to name it "Kevin" because at first, he thinks she is a male for how she looks. Later in the movie, it is revealed that Kevin is a girl. Nevertheless, Russell still calls her "Kevin" long discovering the fact that Kevin is a girl.
Although she doesn't talk in the movie, Pete Docter provided the gobbling sound she makes.