Is capitalization of special words acceptable without another rule to justify it?

I've found that it's a rather contentious issue in the Pokémon fandom whether names like "Pikachu" should be capitalized when referring to the general species (and similarly for certain other words). Such as:

If your team consists of six Pikachu, you're going to have a hard time against Ground-types.

Many people who I've seen talk about the subject believe that those bolded letters should not be capital, because there are no rules to make it such.

However, I've also noticed this "special capitalization" of words in other, actually literary series as well — for example, the Dementors, Boggarts, or Hippogriffs from Harry Potter, or the Yeerks and Andalites of Animorphs fame.

Any arguments I've seen on the subject have invariably degraded into bickering about different sides of the debate. Are there any more authoritative opinions on such a phenomenon, and if so, where can I find them?

(What's special about the Pokémon case is that the species as a whole is usually only referred to by a "characteristic instance" of it — while you wouldn't say "I like dog better than cat because it's more loyal", you could definitely say "I like Squirtle more than Charmander, because it's cuter".)