liquidsnakehpks: (...)go deeper, the monsters differ according to the various places (...)
My bad, I should have stated that I meant distinct kind of monsters as in requiring a different tactics/strategy, or even actually requiring strategy at all. Oh, wait. I did...
I've seen and fought every single opponent in the game, thanks for the link though.
SPOILER WARNING! PLEASE DO NOT READ IF YOU WANT TO EXPLORE ON YOUR OWN!
There are three and only three different monsters in Terraria today, if you ignore variance in Hitpoints and graphics, which simply doesn't qualify as distinctively different to me.
1) You have various incarnation of plain, dumb, jump and run, follow the target creatures, namely Angry Bones, Goblin Peon, Goblin Thief, Goblin Warrior, Man Eater (slight variation here), Skeleton, Slime (including Pinky) and Zombie.
2) You have the many a fly around enemies (which you could group further into ignoring ground or not if it would really make a big enough difference) including Bone Serpent, Burning Skull, Demon Eye, Devourer, Eater of Souls, Eater of Worlds, Eye of Cthulu, Giant Worm, Hornet, Meteor Head and Servant of Cthulu.
3) There are some teleport and shoot monsters like Dark Caster, Fire Imp and Goblin Sorcerer.
And that's it. As for the difficulty, I doubt that you require more than 8 health potions for Skeletron and it shouldn't get you even remotely close to danger. The whole Goblin invasion ended up draining me a whole single health potion, simply because I was getting a bit lax towards the end. The only real threat I found was in the dungeon, simply because of the mass of enemies spawning while trying to open chests and bone serpents pushing you into lava.
Non of the creatures required me to take a different approach from 'stand there and shoot/hit', occasionally dodging an attack.
There's a plethora of possibilities to create a challenging and different encounter with distinct creatures and I'd be happy to iterate on my vision of the concept if you want.