This is, in my personal opinion, a deeply wrongheaded approach. Monsters should not be thought of as belonging to some cut-and-dried taxonomy. Monsters shouldn't just be a collection of known strengths and weaknesses. Each encounter with a being worthy of being called a "monster" should be memorable, and potentially suspenseful.
I could go on and on about this, but fortunately, others already have. Here's what Lamentations of the Flame Princess has to say about the matter, in the "Grindhouse Edition" referee handbook:
You will note that unlike almost every roleplaying game ever, there is no “stock” list of monsters included with this game. Because monsters should be unnatural and hopefully a little terrifying, using stock examples goes against the purpose of using monsters to begin with. Again, this is from an ingame perspective. That the players need challenges and fights is understood, but the temptation is always too great to skim through a standard monster list to lazily fill out an adventure. Don’t do this. Not ever!
Let the scenario and the setting and the situation decide what monster would be appropriate. Take a moment and think something up for yourself. If you do use established monster books, if something jumps out at you, put it aside and design an adventure around that monster that you find interesting. But don’t pull one out simply for the sake of filling out Room 5b on a map. Players can tell when something is inspired and when something is included “just because.”
So avoid lazy monster placement and embrace creative monster invention. As a campaign moves along, a Referee should have a notebook of creatures utterly unique to that campaign.
And here's the DCC take:
A key element of player experience in the Dungeon Crawl Classics Role Playing Game is a sense of wonderment. Your job as judge is to convey “the sense of the unknown” that was so easy to achieve when we were children who did not know all the rules. One way to achieve this is to make monsters mysterious. The less the players are able to predict about the specifics of an encounter, and the more they depend on role-played hearsay, legends, and lore, the more exciting their encounters will be—regardless of monster statistics.
Never describe a monster using a specific noun (e.g., “goblin” or “orc”). Always describe a monster using physical characteristics (e.g., “a four-foot-tall man-like creature with green skin and pointed ears”). Let players learn the capabilities and characteristics of monsters through experience. They can name these creatures as they see fit—but they should name them, not you
Now, I take this advice very much to heart, but I allow for some exceptions. Not every monster type is unique in my campaign. There are dragons, for instance. But still, within each type, I try to include some variation. Dragons, in particular, are awesome and unique creatures, not really meant to be fought (at least, not at this point in my current campaign). A given dragon will have an ancient history, a cunning and varied set of tactics, a personalized assortment of spells, and generally various kinds of preparations in its personalized lair.
What I try to get away from are boring and generic monsters like how D&D treats stuff like Orcs and other kinds of fodder. In my campaign, the weakest generic monster is the Beastman, and they are probably third-level equivalent. A horde of rampaging Beastmen is something to be feared, and descriptions of their wild-eyed ferocity, foaming mouths and bellowing screams are sure to pepper any melee they participate in.
If I need something more generic and weak, I'll just choose something more mundane, like bandits, wolves or barbarians. If you're going to go fantastic, go all the way, and conversely, if you don't need fantastic, don't do just a little.
In addition, there's something I like to do with monsters, and that's to take well-known folkloric archetypes and give them a little twist, without changing their fundamental identity. There's a couple reasons that I do this. Primarily, I like to do this to keep players on their toes, so they can't rely entirely even on their own knowledge of mythology (after all, mythology in a fantasy world is just gossip about magical stuff, and it can be wrong). It also makes things a little more mysterious for players, who can't just roll with their expectations.
Another reason I do this is because a fantasy setting often has some explanation as to why all these very different magical beings would coexist, and sometimes a little bit of work is needed to make them all coherent. This makes it all feel a bit more of a piece, rather than just a grab bag of folkloric figures.
So, that's my general philosophy for monsters. In upcoming blog posts, I'll describe some of my takes on fantasy monsters. These are distinct types, but they are characterized by divergence from strict mythology, as well as variation within type. Note that what little statistics are supplied will be given in terms of my own custom game system, Empress. This may not be familiar to readers, but enough of my notation should be understandable to the casual OSR reader.
Now you have that to look forward to.