Well, actually, they don't have to be animals--plants, fungi, bacteria, chromistans, whatever you think might work would do too!
Game Freak has had a lot of interesting concepts made into Pokémon, such as pangolins, diving bell spiders, sea lilies, pistol shrimp, and Viking cats, but the real world has so many interesting things, and I feel there's still a lot left to go through. Here are some organisms I'd like to see a Pokémon made from sometime:
Betta
It seems odd to me that we've gone this many generations without something based on a creature known as a "fighting fish." Perhaps we're headed in that direction though, with Barraskewda learning Close Combat. Betta fishes have very recognizable, distinctive, graceful appearances (the males more so though), and that famous vicious competitiveness males have with other males.
New Zealand Glowworm
These weird but beautiful creatures create strings of bioluminescent beads in tubes of sticky mucus to lure in prey. Once the prey is ensnared, the glowworm travels across its network of tubes toward the prey and goes in for the kill. A Pokémon based on this creature could be a Bug/Fairy, a more malevolent counterpart to Ribombee.
Virginia Opossom
This is the single best candidate, if you ask me, for that coveted Normal/Ghost type combination. This critter is, well, early-game Normal-type mammal material, but it's famous for playing dead, to the point where it smells like a corpse. A Pokémon based on an opossom could also play dead in the midst of battle, change form, and gain the Ghost-type.
Geographer Cone Snail
Does this snail look like a tank to you? Because it functions a lot like one, though that turret at the front fires a highly venomous harpoon. Cone snails exhibit the fastest movement of any gastropod, able to sit somewhere inconspicuously and shoot debilitating neurotoxin into an unsuspecting victim in an instant. Imagine a super-slow snail Pokémon with a Poison-type priority move...
Ecballium
That is the fruit of the Ecballium, but don't try to eat it, because it functions more like a wind-up cannon. Rather than waiting for an animal to carry the seeds elsewhere, Ecballium shoots them out to long distances by storing up tension in its cells, then releasing them all at once. A Pokémon based off of this would have some very painful Bullet Seeds.
Bladderwort
Each of those little black bits is a death trap, because bladderworts are carnivorous plants. When a small aquatic creature approaches one of those capsules, the plant sucks the animal straight into the capsule, where it's sucked dry of nutrients. I always thought that Pokémon is oddly low on carnivorous plants (it's been just the Bellsprout line and Carnivine so far), though they do sometimes adapt non-carnivorous ones into carnivorous plants (like Cacturne).
Dodder
Even rarer, though is the parasitic plant. Actually, I don't think we've had any yet, just a parasitic fungus (Paras line). The dodder is a plant with no roots and no photosynthesis. Instead, it latches onto other plants and effectively controls them, the host supplying the dodder of its every need. Perhaps a dodder Pokémon could take control of other Grass-types while putting its own change on them in some way.