What is a Cryptid?

Cryptomundo has a featured post at the moment called "just What is a Cryptid and What Isn't?" This is the post by Craig Woolheater:

"This discussion comes up from time to time here on Cryptomundo, most recently in regards to posts about the Slenderman here:
Cryptomundians have strong opinions regarding this.
Cryptomundian Fhqwhgads wrote this detailed summary on what he thought qualifies as a cryptid:
@springheeledjack: To quote something I have written elsewhere: each of the following have been called cryptids by some people.
1. An animal known to currently exist, but not in the location where it was seen. The sightings of big cats in Britain fall into this category.
2. An animal which is known to have existed and is believed to have gone extinct during human history, but for which there are unconfirmed sightings after its presumed extinction. Alleged sightings of the Ivory-billed Woodpecker and of the Tasmanian Wolf fall into this category.
3. An animal which is known to have existed and is believed to have gone extinct before the dawn of human history, but for which there are unconfirmed sightings. Alleged sightings of dinosaurs and pterosaurs fit this category.
4. An animal which is not known to have ever existed, but which is biologically possible and for which there are unconfirmed sightings. Alleged sightings of Bigfoot go here, since Bigfoot seems to be either an ape that has independently evolved a preference for bipedalism or a relative that diverged from us at or about the time of the Australopithecines.
5. A creature which appears to be biologically and/or physically impossible. Mermaids, which make no sense biologically, and werewolves, which appear to violate the laws of physics, go here.

Category 1 is really pretty boring. There is a long, sad history of people bringing exotic pets or specimens into an area outside that animal’s normal range and releasing it, usually with disastrous consequences for the pet, sometimes with disastrous consequences for the local environment.
Category 5, on the other hand, belongs in the category of the paranormal, if not in the category of pure fiction. As with Category 1, many self-professed cryptozoologists do not consider these to be truly a part of cryptozoology.
Category 2, on the other hand, is dubious. Even the most skeptical are not surprised that sometimes announcements of extinction are premature, though some survivals would be more surprising than others. It seems much more likely, for example, that the Ivory-Billed Woodpecker is still in existence than that the Dodo still survives.
That leaves Category 3 and 4 as the solid core of what is meant by cryptozoology, with Category 2 on the edge and Categories 1 and 5 outside the boundaries.

~ Fhqwhgads
I would urge all to weigh in on this topic here on Cryptomundo.
See also:

What do you readers think? What is a cryptid? Here's some of my examples:

Bigfoot - Cryptid, Is said by some to exist, but there is no "definitive proof".

Alien - Not a cryptid. If aliens are real they would not be categorized as a cryptid.

Thylacine - Cryptid, said to be extinct but may still survive.

Do you agree with what should be a "Cryptid?" What are some of your favorites? Let me know in the comments!