Here's one: snakes are books.
Every serpent is a text. Certain people (and non-people) know how to read their scales.
As they grow, the animals revise and expand themselves until they die.
The most common and convenient method of reading a snake (among human ophdiobibliologists) is having it slither through an ivory serpent-reader--a sphere with ornately carved orifices and channels.
Unusually large specimens can be read with the use of specialized lenses.
(And here you thought that was just a Chinese puzzle ball.)
Those who know the spoken language of the Yuan-Ti know what snakes hiss. Each snake is hissing its name--the title of the book that it is.
Common snakes are usually fairly uninteresting works--garter snakes tend to be cookbooks, corn snakes are generally works of adventure fiction with cliche characters or too-convenient endings. Rarer breeds--100' anacondas, albino cobras--often contain long-forgotten secrets or comprise unique works of poetry or philosophy.
Since snakes are natural phenomena, and all books are, in one way or another, discussions of natural phenomena or its effects, snakes could be considered a continuous monologue that the world produces about itself. Thus the symbol: a serpent eating its own tail.
Giant snakes are typically encyclopedias or great multi-volume sagas representing the myths and theogonies of entire cultures.
Scholars disagree: the amphisbaena is either a palindrome or a work which reveals an entirely different (yet equally coherent) narrative when read backwards.
Nagas are linguistic texts, translating from the languages of snakes to the languages of humans.
The snakes growing from the heads of medusae are generally reference works and the medusaes themselves are often cataloguers--tending private libraries containing nothing but caged snakes,, selectively breeding exotic and daring new works.
The Librarians--known in the east as Yuan-Ti--also catalogue and breed works, though in a far less dilettantish and casual fashion--they believe that careful control of cross-species breeding can and will one day unveil a Great Glistening Book containing all the secrets of creation. Each Librarian is a visionary religious work attesting to the perfection of one or other path to the Great All-Serpent.
Mariliths contain terrible secrets and blasphemies.
It is said that beneath every great library in human civilization a cabal of wizard-scholars tends to a chained Lernean Hydra. They carefully transcribe and translate the information gleaned from the beast's skin before pruning off each head in turn and reading what grows in its place, thereby nurturing a constantly updated stream of knowledge.
Dragons are books of magic spells. Owing to the difficulty of reading them while alive, complete dragon hides will almost always fetch a higher price from the right sorcerer or alchemist than from any armorer.
Pseudodragons are helpful but incomplete summaries of the contents of their larger brethren.
I like this'cause it explains and rationalizes how every other monster is part snake, fits with ideas common to all kinds of cultures equating snakes with knowledge, and gives the party some pretty all-purpose adventure hooks, whether they want money, power, knowledge, or all three.
59 minutes ago