Hi. My answer begins with a question, what do you mean by a friend? If it is a soul with loving caring and sharing attitude coupled with a sense of responsibility and understanding of virtues and vices, than it should be a man. A man can be a man's best friend. It's a prerequisits of social obligation with which a person is recognised. As the saying goes, "show me your friends and I will tell you what you are" - As far other animals and creatures are concerned, I believe that everything has been brought into existence to serve the human kind. We should love them and treat in as good manner as possible, but they can't be regarded as our friends. If anyone, owing to helping nature, regards an animal as his friend than why can't he be his relative?
Similarly there are many creatures that are more beneficial to us in one or the other way, sun, moon, water, trees, vehicles...then why is that no relationship is mentioned with these? The matter is simple relationship of socially obligatory nature must be among humans where as with the rest of creatures we should have loving and caring bonding.