We don't have to be alike to be friends.
Leaving the absolutely adorable qualities of this picture aside, what can these two critters teach us?
Well, the most important quality for friendship is making the effort. These two buddies aren't the same color, size or species. They eat different things and have a different view of life. But if the cat jumps up on the package of compressed shavings and the horse puts his head down, they can exchange a friendly nuzzle and/or head bump.
Befriending people of different backgrounds, interests and beliefs opens windows if not doors. If you won't be friendly with someone unless they are an exact duplicate of yourself, you don't want a friend, you just want a mirror to admire yourself in. I especially value those friends who may have different political or religious beliefs or who have differing life experiences. They never fail to give me a window into their world, which can be valuable for a writer who wants her characters to be diverse, entertaining and believable. My friends introduce me to new interests, ideas and hobbies and make my life more interesting.
All that might be obvious, but there's one more aspect to consider. If you are the cat, be careful around the horse's hooves, because horses can't see what is under their noses. If you are the horse, don't let the cat sleep on your back*--not that there's anything morally wrong with that arrangement but if the horse spooks and the cat digs its claws in than there may be breakage in the stable.
In other words, be respectful of the innate differences and then be friendly.
*Yes, I knew a horse that permitted his cat buddy to do this.