Wow. 16 seems like a stretch. Is that in total? A "no more than 16 per category" I could probably see. Does that/is it supposed to count PC species/races OR any/all non-human species/races (or none)?
I'll give it a shot...but if I go with what I have already established to exist in my homebrew....I'm pretty sure we get way past 16.
Let's see...hmmm...Well, let's start at some of the first things at first level and go from there...
Monstrous Humanoid Species
1. Kobolds
2. Goblins: standard goblins
--Puckwudgie (monkey-tailed jungle variant),
--Grindylow (aquatic variant)
--Boggle
--Redcap (evil fae variant)
--(2a) Froglins (evil amphibious mutated goblins, created in the ancient past {
see Gorgons., below}, bullywug replacement)
--2b) Bugbears
3. Orcs: black (subterranean/mountain/wasteland), green (surface-dwelling alpine woods/valleys), and blue (ice/tundra orcs)
4. Trolls - which includes the typical green rending male of the species and Hags (the females of the species)
--Forest/"Green" Troll & Green Hags
--"Rock Trolls" (a.k.a. "Ogors" in the original dwarvish

& Black (Annis) Hags
--"Aquatic Trolls" (Aughisky) & Sea Hags
--Oni (extraplanar/"ruling class" trolls, the only male trolls who can use magic) & Night Hags
5. "Gorgons" ( the remnant mutates of a fallen empire dedicated to/cursed by an unnamed/forgotten reptilean/ophidian demonlord)
--Lizardmen (the "common folk"/mass population of the empire, reduced to "dark age" type primitives with little-to-no memory of the heights of their depraved but advanced former reptile-civilization)
--Medusae ( the priestly caste, male and female, of the fallen Gorgon empire)
--"Snakemen" (the "nobles" caste, ophidian half-man/half-serpent, Yuan ti abomination replacements)
6. Grorn (D&D gnoll replacement, more canid/wolf/jackal men than hyena, still demon worshippers, barely more than feral, relentless tracker/hunters, eat anything - even orcs and lizardmen fear a grorn tribe in the area)
Giants & Giantkin
7. Giants
--7a) Forest Giants (ent/treant replacement, Neutral)
--7b) Hill (mostly Evil, some Neutral can be tricked/bribed/or bargained with into being helpful)
--7c) Stone (Neutral, some Evil or Good individuals)
--7d) Frost (Evil)
--7e) Fire (Evil)
--7f) Sky/Cloud (mostly Neutral, some Good, some Evil, believed to be myth)
--7g) Sea/Storm (Good, believed to be myth)
8. Giantkin,
--8a) Firbolg (Good, old school celt redhead forest-dwelling giant warriors with magical abilities)
--8b) Cyclops (Neutral -but quick to anger- island cave-dwelling)
--8c) Fomorians (Evil, monstrous shadow-evil-fae-affiliated, mortal enemies of the firbolg)
9. Titans - the origins of all giant species. Mythic celestial creatures, believed to exist only in the world's ancient pre-historic (pre-elf/dwarf/human) past.
10. Beasts, Monstrous/Magical. I kinda feel like this should just count as one. You can have creatures from tiny harmless librarian "bookworms," giant speaking hawks, to multi-headed fire-breathing man-eating giraffes (hell, even just dire rats or wolves/wargs!) that are all just standard "fauna" in a magical world. This is just a sampling of some of the "animals" that are a bit more than just "flavor" trappings of the world.
--Griffons
--Silverfawn (white elk/stags that can cover great distances and teleport like unicorns)
--Elfcats (cougar to tiger-sized grey-green mottled or striped "forest cats" with magical abilities)
--Moorcats (D&D displacer beast replacement)
--Demonbats - fire breathing wolf-headed bats.
--Blackwood Beasts (magic-mutated chimeric creature, bites like a lion, claws/hug like a bear, constrict like a serpent, scorpionic tail-sting and venomous spittle, gorge with horns/antlers, big enough to crush/trample, all nasty animal things in one critter)
--Bonnecon (D&D bull-gorgon replacement, more monstrous horned caprine or antelope than bull. I made it acid cloud-breathing instead of turning you to stone)
12. Undead. (I know this isn't all of 'em. But the ones I've actually used/had in the world. This seems like enough.)
--12a) Skeletons
--12b) Zombies
--12c) Ghouls
--12d) Shadows
--12e) Wights
--12f) Wraiths
--12g) Mummies
--12h) Vampires
--12h) Death Knights (believed only legend/none known)
--12i) Liches (only 2 known in the current world, myths/legends of others)
Dragons & Dragonkin
13. Dragons
--13a) Dwimunduun (chromatics): White, Black, Yellow, Green, Blue, Red, Purple
--13b) Glimmariin (metallics): Brass, Copper, Bronze, Iron, Silver, Steel, Gold (exist only in myth/stories, to anyone's knowledge)
--13c) Astraeliin (planar): Faer(-ie), Gloom, Shadow, Prismatic ("Rainbow"), Ethereal ("Mist"), Astral ("Star")
--13d) Vaetalaer (gemstone, only singular individuals of each remain): Topaz, Onyx,"Crystal" (Quartz), Amethyst, Sapphire, Ruby, Emerald
14. Dragonkin
--14a) Lapdrake (standard D&D pseudo-dragon replacement)
--14b) Beithir ("Behir")
--14c) Hydra
--14d) Linnorm (serpentine non-winged, forelimbs only)
--14e) Drakion (D&D dragonne replacement)
--14f) Wyvern
15. Elementals, as a general rule, must be brought by magic into the world (or find their own way in from their plane, if they desire. But why would they want to?!). So, for the most part, they don't count as "in the world" creatures with a few noted exceptions:
--15a) Salamanders, the fire elemental serpentine men have a stable gate to their plane and claim a large section of desert called the Flaming Sands as their own territory. They are not found (and quickly die, unless kept somewhere comparably hot) anywhere outside of this territory.
--15b) Ifrit, the fire-based genies flow in and out of the portal(s?) found in the Flaming Sands., occasionally moving into other of the Seven Deserts that comprise the desert empire of Thel to cause trouble and harass caravans (pilfer treasures). It is believed a powerful Ifrit sheik is setting itself up as a ruler over the Salamanders and other random elemental denizens (and fire giants!) of the realms' hottest desert - literally, burning, aflame - flaming geysers, fire-"nadoes," conflagration "fire-sand storms," etc...
--15c) Wyrds. The typically conjured elemental guardians, typically of a serpentine form -water wyrds being most common, but other types/shapes are known to exist- can by from any of the known elemental planes and serve as guards/protectors/alerts for special sites, items, or persons. Among Thel's priestly-vizier caste of elementalist magi, wyrds of their preferred element are used as (roughly equivalent to, it is understood among the arcane classes,) a non-Thellic wizard's "familiar."
16. The Obliterax. ...You do NOT want to run into one of those.
Aw crap. Forgot about the Fae. Well, like elementals, we can go with they are not "in world" creatures but must be brought or find their way into the prime....granted, with far greater ease than elementals can (there are many more "thin places," if not active/permanent "gates," between the veils of Faerie and Orea's "Prime Material").
Of course, if I were to include Player species, then we have to add:
Humans, Elves (6 kinds),
Dwarves (5 kinds),
Halflings (currently, 3 kinds),
Half-Elves, Gnomes (5 kinds, most of which are not known to the general world),
Horken (half-orc/hobgoblin replacement),
Zepharim (winged humanoids - think Thanagarians, not Aarakocra),
Jerali (felinoids - think Thundercats, not Tabaxi),
Ossan (ursine humanoid shapeshifters, not exactly "werebears")
,Centuars, Satyrs, and
Sprites.