I had a Rock Climbing Rattlesnake once. Twice the size, and had a climb speed on even near vertical cliff faces. Great for chasing PC that are trying to mountain climb.
There was also a Tyranthraxus Spider. Very small, hard to hit, high AC. And a incredible jump range (like 20 feet) so it could jump from brush onto PCs. (deadly poison, scaled to kill most 0 level NPCs, hence the deadly rep, but still a threat up to 3rd levels)
I think a person on these forums inspired me to create a very flat "Bookscorpion". Standard scorpian, but you did not expect it inside your old book.
Real animals have what I call "palette swap" issues. Real animals are not interesting in combat. They are mostlyeither tough brutes, fast brutes, strong brutes, flying brutes, swimming brutes, or brutes with extra attacks. They are much like the same 4e-style stat block leveled up or down and given a level and given an movement quirk or extra senses. 3 versions of the same bears, canines, raptors, and felines.
That being said, there are somethings you can still do. There are lesser used animal families like weasels, hippos, skunks, mongoose, and large lizards that are not heavily used.
Armor Class 15 (natural armor) Hit Points 126 (11d12 + 55; bloodied 63) Speed 0 ft., swim 120 ft.
Saving Throws Str +9, Con +8 Skills Athletics +9, Perception +3, Stealth +3 Senses Blindsight 60 ft., Passive Perception 13 Languages -- Proficiency Bonus +3 Maneuver DC 17
Aquatic. The shark has advantage on attacks against creatures in water that do not have a swim speed. Additionally, while submerged in water it has a Dexterity of 16 (+3) and gains a +6 bonus to Dexterity saving throws.
Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.
Water Breathing. The shark can breathe only underwater.
ACTIONS Bite. Melee Weapon Attack:+9 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) piercing damage. If this attack is not made after a charge, the target is also grappled. While the shark has a creature grappled if cannot make a bit attack against another target. If the shark moves at least 20 feet straight toward the target immediately before the hit, the target takes an extra 16 (3d10) piercing damage.
Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., one target not grappled by the shark. Hit: 18 (3d8 + 5) bludgeoning damage.
REACTIONS Chomp. If a creature ends its turn grappled by the shark, the shark can make a bite attack with advantage against the target.