Imaginary friends that children have are human most of the time, since humans are what humans relate the most to. However if the kid has a favorite animal or tv show the imaginary friend will often be inspired by that. Sometimes they will be humanoid, like they'll be a person... but they're purple. Or something like that.
Basically, anything a kid can like can become an imaginary friend.
A small addition that only applies for the openminded in this discussion would be imaginary friends that supposedly turn out to be actual ghosts. Dead relatives the child never met and has never seen or heard of, for example.