[S]eeing and understanding the skeleton and muscles is one of the keys to learning how to draw a deer. I recommend learning six muscles that make prominent bulges beneath the skin and fur. Muscles tend to originate from larger stable bones, cross at least one joint and insert into another bone that usually is smaller and further from the body core (distal). Muscles also tend to have most of their mass closer to the body, getting thinner as you go out on the legs.
The deer has an ungulagrade stance, standing on its toenails or hooves. The bones of the instep (metatarsals) and the bones of the palm (metacarpals) are fused together to make one strong bone. This puts the heel and wrist joints high off the ground. Unlike dogs and cats, the metacarpal bone is about as long as the forearm. Note how most of the bulk of the muscles is carried close to the core of the body. The legs get thinner at each successive joint and the muscles of the hind leg are larger than those of the front leg.
Print out the mammal anatomy worksheet and follow the In the step by step guide below, adding one muscle at a time. Then envision how these muscles show through skin and fur. This will help you memorize the shapes and locations of the muscles better than just reading this post. Click on the first image to start a sideshow with step by step details.