Jerome K. Jerome in the preface to his comic novel Three Men in a Boat tells us that his characters are not imaginary, but 'things of flesh and blood'. The best I can do so far is the phrase sunt natura visceri et sanguinis, but I wonder if there is a better, preferably attested classical phrase for this?
I have found classical evidence for your own wording.
In an extended argument for the improbability of imaginary beings such as Centaurs, Scylla, and the Chimera, Lucretius contrasts these with lions and other real-life beings:
Flamma quidem vero cum corpora fulva leonum
tam soleat torrere atque urere quam genus omne
visceris in terris quodcumque et sanguinis extet …
Indeed, seeing that fire is accustomed to scorch and to burn the tawny bodies of lions as much as every kind in the world that consists of flesh and blood …
Lucretius, De Rerum Natura, 5.901-903