Spoiler warning!
You might want to skip down to the next heading if you do not want facts about the game's storyline or plot revealed to you.
The forest and plains act; Act I contains the weakest of all the enemies. It isn't so much themed as really diverse in what creatures reside in it. Demons, skeletons, and beasts are all pretty equal in terms of population, however each is more common in various areas (this is common to both Diablo and Diablo II).
Zombies >> Hungry Dead
Zombies are the second weakest enemy in the game, they are slow, they have a much higher maximum health than quill rats, and have a strong (again slow) melee attack. Unique zombies can sometimes be a hassle depending on their special traits, i.e. cold enchanted and extra strong can cause them to be over powered.
Quill Rats >> Spiked Fiends >> Thorn Beasts >> Razor Spines
Fallen >> Carvers >> Devilkin >> Dark Ones
Fallen Shaman >> Carver Shaman
Gargantuan Beasts >> Brutes
Gargantuan Beasts can be found in the Blood Moor, and Den of Evil. Brutes are found within the Black Marsh.
Skeleton >> Returned
Skeleton Archers
Dark Ranger
Dark Hunter >> Vile Hunter
Vile Hunters are found in the Dark Wood.
Dark Spearwoman
Moon Clan >> Night Clan
Misshapen >> Tainted >> Plaguefiend
While Act I may be an environment that is very grassy, Act II is much more arid and desert like. The creatures found here range from sandy beasts to mummified undead and finally to anthropomorphic snakes and cats.
These are the basic type of this enemy. They look like a skinny frog with a rough, almost rock like hide. Cliff Lurkers and their successors are very quick and are characterized by their leap ability, which they use randomly and only sometimes for attack. Their attack is much lower than other enemies, but they have a high amount of health.
An aggressive species of man-like snakes. Unlike normal serpents, Claw Vipers slither upright and have arms.
There are ones with whips, ones with javalins, and ones that toss poison gas containers.
Sandworms >> Rockworms
Scarabs/Death Beetles
Flayers >> Undead Flayers >> Undead Soul Eaters?
Flayer Shaman
Frogs >> Slime Princes
Sextants >> Cantors >> Heirophants
Just like the basic zombies, although with much higher damage and health.
Flesh Beasts >> Stygian Hags
Flesh Spawners >> Stygian Dogs