I really do think I would be a cat.
I like to be alone (true introvert here) but I enjoy quiet company at times. I can have my guard up when I meet new people, but am actually quite a goofball once you get to know me. I can appear as quite serious and aloof (especially given the career I have chosen) but I am actually a very unserious softie at heart.
I also like to spend my days eating and sleeping. Preferably in a warm sunbeam or in a cosy pile of blankets.