The question I want to ask is: how great is the scope for a writer to bend the rules regarding traditional/mythical creatures? (Note: I mean, where such creatures don't form a sacred part of some group's living belief system. I don't mean to get into the issue of cultural appropriation just here.)
It seems to me that several factors come into play: a) the number and variety of previous textual (in which I include film and TV) interpretations; b) the general fame of said mythical/traditional entity; c) the extent to which the entity may be considered to be in a moribund condition, and in need of revivication (see Zombies, below). Let's consider some specific cases...
a) Vampires have a
b) Other creatures are less well known. Boggarts, for example. JKR's boggart in The Prisoner of Azkaban was unsuccessful in my view, at least as a boggart. It's not that the idea of a shape-shifter that takes the form of one's worst fear is a bad one in itself, but it's so different from existing boggart tradition that it breaks rather than plays with the rules. Nor have boggarts had the lengthy and various literary exposure that would allow such a non-standard version to be absorbed without indigestion. It bothers me that there are now millions of people who think that JKR's version is what a boggart is.
c) Having said that, a well-established and seemingly-stable set of features can sometimes be overturned by a sudden evolutionary leap, which is what appears to have happened in the last decade with the appearance and spread of fast zombies. Their success suggests that perhaps there was something inadequate in a deadly menace with a top speed of 1.5mph.
Is it possible to draw these random observations together into something more general, I wonder? Are there rules that you set yourself, or those whose books you read, regarding the extent to which supernature can be monkeyed with? Or is every case unique?