Astrology sorts its twelve signs into four elements, three signs apiece. I did not plan the bestiary around that, because I built it one sign at a time. When I finally laid all 192 out together, the elements had organised themselves without me.
Fire: Aries, Leo, Sagittarius
These are the bestiaries that move across open ground. Wild sheep on cliff faces, cats on savanna and mountain, horses on plain and steppe. Nothing in the fire signs hides for a living. Even the most secretive animals here, the marbled cat and the Pallas's cat, are ambush hunters rather than concealers, which is a different thing.
What they share is commitment. A bighorn ram running at another ram. A cheetah opening up across grass. A Przewalski's horse crossing steppe that offers no cover whatsoever. These are animals whose whole strategy involves being seen and being fast enough that it does not matter.
Fire is also where all the mythical horses ended up. Sagittarius holds a unicorn, a pegasus and a kelpie, more legendary creatures than any other sign. Horses attract myth more readily than almost anything else we have shared a landscape with.
Earth: Taurus, Virgo, Capricorn
Weight and persistence. Wild cattle that will not be moved, harvest animals that store and sort and prepare, mountain goats holding ground at altitudes where the air stops cooperating.
These are the slowest bestiaries and, I think, the most quietly moving. The aurochs that every domestic cow descends from held on in European forest until 1627. The saola was unknown to science until 1992 and has been photographed alive a handful of times since. The kouprey may already be gone. Earth signs quietly accumulated the animals that persist and persist and then, sometimes, stop.
Virgo is the exception. Its bestiary is birds, insects and small mammals rather than anything heavy, because the sign is about discernment rather than endurance. The owls, bees and foxes in Virgo have more in common with the air signs than with their fellow earth signs. It is the clearest case of a sign's own symbolism guiding the choice over the neat grouping.
Air: Gemini, Libra, Aquarius
Two of the three are birds, which is the obvious move. Gemini's mimics and mirrored moths, Libra's hoverers and balanced fliers. The third is not birds at all.
Aquarius went to the deep sea. The Water Bearer is an air sign carrying water, which is a strange enough proposition that the usual birds felt too easy. The deep ocean is where evolution stops behaving predictably: animals that make their own light, animals with transparent heads, colonies of specialists that behave convincingly as a single creature. If any sign should hold the genuinely alien rather than the merely unusual, it is this one.
What the air bestiaries actually share is not habitat but pattern. Animals defined by the relationship between their parts: two wings that must match, two voices that answer each other, a siphonophore that is many creatures agreeing to be one.
Water: Cancer, Scorpio, Pisces
Only two of the three are aquatic. Scorpio is scorpions, spiders, snakes and lizards, almost all terrestrial and most from dry country.
That was deliberate. Water signs are associated with intensity and with what stays below the surface, and Scorpio takes that furthest. Its animals hide what they carry. A deathstalker scorpion is four inches long and gives no external indication of what it is capable of. Nearly every animal in the Scorpio bestiary is defensive rather than aggressive, which is the part most people get wrong about the sign. There is a whole essay on that here.
Cancer and Pisces are properly marine. Cancer collects animals that build, borrow or abandon shells. Pisces collects animals that cross between conditions, and ends on a selkie, which is a seal until it decides otherwise.
How much this is worth
Not much predictively, and I would rather say so than oversell it. The elements did not drive the choices; they emerged from them, and where the grouping conflicted with a sign's own symbolism, the symbolism led. Virgo is light rather than heavy. Scorpio is dry rather than wet. Aquarius has no feathers.
What it is good for is reading sideways. If your animal sits in a fire sign, the other two fire bestiaries hold creatures solving problems the same way yours does, and that comparison is often more revealing than the sign you were born under.