Snakes are a rather small and typically weak species of animal. However, some are quite deadly due to their poisons.
Most snakes are non-aggressive without provocation, but some are quite hostile and will attack first.
Snakes have many variations from your standard snake to to the much larger Killer Snakes.
As they are animals most worlds contain snakes. As such the following list will be quite long.
Worlds that are known to have various forms of snakes are as follows:
Interworld System of Planets
.hack
Bleach
Digimon
Final Fantasy Universe
Gundam
Hell
Pokemon
Appleseed
Blood
Card Captors
Code Lyoko
Dark Cloud
Dead or Alive
Dragon Quest
Dragonball
Duel Masters
Enchanted Arms
Eureka Seven
Full Metal Alchemist
Ghost in the Shell
Hellsing
Infamous
InuYasha
Magic
Naruto
Ninjago
Neon Genesis Evangellion
Oblivion
Phantasy Star
Purgatory
Earth
Sailor Moon
s-CRY-ed
Shadowverse
Star Ocean
Suikoden
Super Sentai
Yo-kai Watch
Yu Yu Hakusho
Yu-Gi-Oh
Zatch Bell
Code Geass
Growlancer
Karas
MAR
Rosario + Vampire
Sekirei
Trinity Blood