Every edition of D&D has called any hostile NPC a "monster", including human opponents. There is even an explicit note in 1st edition AD&D Monster Manual that the term is used this way for both human and inhuman opponents (emphasis original):
The term "monster" is used throughout this work in two manners. Its first, and most important, meaning is to designate any creature encountered — hostile or otherwise, human, humanoid, or beast. Until the encountering party determines what they have come upon, it is a monster. The secondary usage of the term is in the usual sense: a horrible or wicked creature of some sort. Thus, a "monster" is encountered during the course of a dungeon expedition, and it is discovered to be an evil high priest, who just might turn out to be a monster in the other sense as well. Note, however, that despite this terminology, humans (and such kin as dwarves, elves, gnomes, half-elves, and halfings) always use the matrix for humans when attacking, even if such humans were encountered as "monsters" in the course of an adventure.
(Note how the term is very loose, and very broad, covering everything; but it also does not have rigid rules effects either, with human and demi-human NPCs using some of the so-called "PC rules".)
The Basic D&D list of monsters has entries for bandits, berserkers, and other humans the PCs might face. The AD&D 2nd edition Monstrous Manual includes dwarves, elves, humans of various sorts alongside more monstrous creatures as "monsters". 4th edition explicitly defines "monster" to mean:
A creature controlled by the Dungeon Master. The term is usually used to refer to creatures that are hostile to the adventurers (often including DM-controlled characters). See also adventurer, character, and creature.
So yes, monsters are different from PCs in D&D, but not different from humans and other humanoid NPCs. They have pre-made stats, but they can always be adjusted by the DM – in some editions there's no guidance for how, so the DM has full freedom to determine their own method and can just assign stats or use PC rules to modify the monster. In other editions that have explicit guidance such as 4e, the rules for improving monsters are very different from PC rules. The only edition of D&D where monsters use vaguely the same rules as PCs is 3rd edition, and even then, the rules make a distinction between PCs and NPCs/monsters where some options that human "monsters" can take are explicitly not permitted to PCs.