If a game is supposed to be a series of interesting choices, environment of the game should offer those choices. And one of the defining features of the game environment are various creatures that inhabit it.
Super Mario Bros. 3 is one of my all time favourite games and one that implements concept of unique creatures rather well. I’m not going to go through and analyze each and every of them, as there are quite a bit. Instead, I’ll pick and highlight some that I find particularly interestingly done.
World of Super Mario 3 is divided into 8 different regions (or worlds), each of them having a unique theme and themed monsters. All kinds of aquatic creatures play big role in water world, while ice world is frozen and particularly slippery. Having monsters and areas themed keeps them coherent and limits amount of different types of monsters that player has to tackle at any given time.
Goomba is probably the first enemy encountered, no matter which Mario game you’re playing and it’s pretty standard: walks from one direction to another, falls from ledges and doesn’t do anything else. But even such limited enemy can offer interesting choices. For example, if player manages to jump on top of several enemies without touching ground in between, the points rewarded are higher. Repeat trick for long enough and instead of points your start getting 1-Ups. Goombas are the first enemy I remember trying to do this, as they often walk in groups. Later levels mix up things a bit by introducing flying goombas, giant goombas and goombas that can jump (the famous Kuribo’s shoe), but the basic premise stays the same.
Some other monsters build on the mechanics of goombas. Koopa troopas (those turle looking creatures) patrol similarly from side to side, but they add extra tricks. A koopa isn’t killed by just jumping on top it, merely flipped on its back. If left alone long enough, it will straightent themselves and continue patrolling. The koopa flying on its back can be kicked forward and then it acts as a projectile dangerous to monsters and players alike. If player chooses to dispatch koopa troopa by using a fireball, the shell disappears.
Buster beetle has similar mechanics than koopa, with one major difference: while koopa can be dispatched by throwing it with fireball, buster beetles are immune against it. This makes them slightly more dangerous to player.
Sort of final evolution of the theme is spiny. They look like red buster beetles with spikes on their back. Unlike buster beetles, spinies can be dispatched by fireballs, but jumping on top of them is dangerous under normal circumstances. Thus, they are sort of like mirror image of buster beetles.
So here we have 4 different monsters (and a pile of different variants of them that I didn’t go through). They all build upon same mechanics, with slight variations: all of them patrol and their main attack is touching the player. Simple variations like this can create nice and interesting situations. For example, place several koopa troopas in a small space between pipes with some bonus item. If player chooses to try and retrieve the item, they have to be mindful of the shells that start flying back and forth rather easily.
Or have a section where goombas are predominant enemy, with some spinies mixed in. Again, player has to be mindful where they jump, unless they want to land on top of some painful spiny spikes.
Or place a koopa troopa in front of some long queue of enemies, inviting the player to kick the shell and run after it, effectively using it as a battering ram. For extra naughtiness, place a pipe or some other obstacle at the other end that sends the shell back to the player.
The point is that you don’t have to have that different monsters, as long as they’re different enough, to create interesting game play mechanics. But on the other hand, they have to be different enough, so they don’t just feel like clones of each other with slightly different stats (imagine regular goomba and turbo goomba that moves at 110% speed).