Belonging to a class of mythical creatures, the specific animal was originally from an English piece of fiction from over twenty years ago (but possibly re-used since). Not a heraldic animal, nor a monster, not from Tolkien, not Scandinavian.

Note : English language but not necessarily English. And as I put it "belongs to a group that is based on creatures of myth" but that won't help much.
(Bismarck) Doggy? NO
(Bismarck) A song? YES (more of a by-product) and at least one other obscure and dreadful song that I know of.

Reckon the animal bit us the wrong track. There's bet free that aren't heraldic, or monstrous.

(Bismarck) Human? YES (in the broad sense of being human).
Audience give a hearty cheer.

(blamelewis) Chaucer? NO   American English? YES

(blamelewis) Ghostly? Woo! NO

(Bismarck) Folky? NO
Just to confuse: I've found that the AOTC was a ghost on one occasion but I doubt that will help anyone here.

[Superman] 20th Century? YES!

Is this a human that turns into another creature?

*Some of the audience hums at the mention of transformation*
(KagomeShuko) Human that turns into another creature? NO but within their history, the AOTC has had a few methods of "changing" :)
(Bismarck) Science fiction? Hmm, there are many Sci-fi elements that come into this, so it is involved, yes.
I thought this could be simple but I forgot the AOTC has had so much variation over the years.