As professional pet photographers, we spend so much time on getting the exposure on our subjects right, we sometimes forget that getting it "wrong" can make a terrific image as well!
Technically speaking, a silhouette image is an under-exposed picture as you would normally photograph the dog or horse and expose them correctly versus the sky. In a silhouette image, you expose for the sky and let your subject go dark and for this reason, it's important that you pay attention to the angle of the subject to ensure you get enough detail in so that your viewer isn't left wondering what the subject is. You'll need to be able to use your camera on manual to pull this off because automatic settings will force your camera to expose for your subject.
The majority of my pet portrait clients order custom albums so I always include a lot of variety in my pet, equine and equestrian portrait sessions and when possible, silhouettes are always on my wish list. Lake Grapevine is a popular pet portrait session location and it lends itself well to silhouettes for dogs. But you don't need to be outside to capture a great silhouette, I also create these in the studio which can be really fun when you toss a ball or treat. I just realized that I've yet to create a cat silhouette in the studio, so I've added it to my personal project list to create.
It's also important to note that silhouettes don't have be to be devoid of all detail. Sometimes a hint of detail, color or texture can make the image more interesting like the studio silhouette of the scruffy dog below, his fur retains some of the golden color and is a bit brighter around the edges and I think it encourages the viewer to imagine what he looks like. Any time you have a viewing spending more time looking at your image, that's a good thing!
(click on any image to view full screen)