The 456 (pronounced 'four-five-six') was an alien race that negotiated secretly with the British government in 1965, and again in 2009. This designation was given to them by humans based on the radio wavelength they used, and they took on the name as their own in their dealings with humans rather than offering the actual name of their species, if any ever existed. As they later revealed, they were drug dealers. The 456 used children as "the hit" because the bodily chemicals they create "felt good". Very little was known about the 456's biology and anatomy, as humanity only had contact with one member of the species, who remained within a nearly opaque gas, contained within a transparent chamber. The natural design of their body consisted of three "heads", connected to one body via three long necks, possibly a conjoined triplet species. It resembled a large, plucked, three-headed swan.
The show's been on forever. Some species names are simply going to get reused. One of the most obvious are the Vogans. Most fans remember the Vogan from season 12's Revenge of the Cybermen. But there's also the [[Vogan (The Vogan Slaves)|Vogans that plagued the Third Doctor… ]]
The top-level category for species is category:species. Obviously, that's hard to grasp, but you'll get there in the end.
If you like alien species that are based on Earth animals, you're in luck. All you have to do is go to the page of an Earth mammal and check out the "looks like" box. For instance, if you wanna know what aliens look like elephants, just go that pachyderm's page and find yourself a breadcrumb at the top of the page.