Becuase of the look, or if it is dragon, dark, steel, or ghost type I will espacialy like it. But mainly becuase of looks and sometimes the stats and move pool, or if it has a ninja/assassin/warrior/ dark holy or evil feel to it
Edit: few pokemon (Other than legys), Zoroark, alkazam, lucario, milotic, haxsouruas, gengar, wevile, empoleon, inferape,typhlosion,sandslash, bisharp, hydredrion,zangoss,pikachu,skymory, milotic, gyarados,scizor, arcanine, houndoom, whismcott,The evolution of dusion,sycther, ninjask, gallad, and guardivor, just a few.