Why do we say "this is a cat" or "this is a forty spotted pardalote" but "this is velociraptor" with no the or a?
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We don't. A search for "this is a velociraptor" returns twice as many results as "this is velocipaptor", and of the latter many results are actually:
Only a very few uses match what you describe, and they might well have been typos.
It certainly is used in the likes of Discovery channel programmes, but there it is for effect. Here, using it as if it is the name of a given individual dinosaur adds an immediacy and sense of closeness as they go on to describe its qualities; generally focusing on the damage it could do to prey (which says as much about the Discovery channel as it does about velociraptors). It also highlights the idea that the technology used to reproduce what a velociraptor looked and moved like is "introducing" you to one.
And it does work, in my opinion and is hence a bit of poetic license worth taking. It doesn't work as well with things you are familiar with, with shorter names, or with multi-part names ("This is great white shark" doesn't work as well as "This is shark"). But it can still be used with them, or indeed with anything, and it sometimes is.
(With more familiar things, rather than add a sense of immediacy, it can give a sense of distance and seeing afresh. I can recall a British nature show that used "this is cat" in examining the anatomy of a cat, and it did give an impression of "seeing for the first time" a very familiar animal).
Normally though, we use a/an and the with prehistoric animals.
The only species name that is more often found without an article than with is man, where the lack of article highlights that we are considering people as a species rather than as individuals. This is a matter of the species name matching one we have such heavy use of in other contexts, and is also less common now that a human would be preferred in most cases of talking about our species.
Because the thing being described is not a velociraptor but a trace (or perhaps even an inference drawn from that trace) left by the presence of one of those animals at some time in the distant past.
If I ever encounter an actual living specimen of a "prehistoric" animal, you may be assured that I will say. "Oh look, there is A velociraptor."