Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny snake. The swarm can't regain Hit Points or gain Temporary Hit Points.
Bites. Melee Attack Roll: +6, reach 5 ft. Hit:8 (1d8 + 4) Piercing damage—or 6 (1d4 + 4) Piercing damage if the swarm is Bloodied—plus 10 (3d6) Poison damage.
Use these stat blocks to represent the creatures they're named for or other similar creatures. For example, the Panther stat block can also represent a mountain lion, while the Giant Goat stat block might represent a buffalo. Any of these stat blocks might also serve as fantastical animals with distinctive names and cosmetic details unique to your D&D adventures.