Either orientation can be used in wildlife photography.
Portrait refers to a photo taken in vertical orientation while landscape refers to shots where subjects are too wide to shoot with a portrait orientation. It’s kind of self-explanatory: landscape is useful for taking photos of landscapes and scenery, while portrait is a useful orientation for people. Either orientation can be used in wildlife photography. Landscape can allow to fit an animal into a scene, giving context to your image. While portrait can capture heads and eyes nicely, giving emotion and story to an image.