Animal colouration and camouflage in relation to habitat is always fertile ground for composition because contrast is eye-grabbing. While an animal's colour often contrasts with the colours around it we still find the scene aesthetically pleasing because we recognise it as natural. Another attractive opposition in these naturalistic scenes is structural. The contrast in the kinds of lines and shapes comprising the animal versus their surroundings break up the image and make it easier for the eye to take in the composition.
Contrasts aside, wild animals make for incredible subjects. Whether they are predators, prey, or both, most animals are ready to move at the slightest sound. This readiness is reflected in their musculature and of course in their eyes. If an artist or illustrator is able to capture this well they infuse the composition with potential, causing the viewer to anticipate what will happen next even within our own muscles.
My favourite way to paint wildlife however is through action packed scenes in which two or more animals are interacting with both each other and their environment. Our most emotive experiences are interactional and I imagine the same is true for all creatures. I'm probably just projecting, but regardless, I think that this is why these scenes make us feel the most.