I'd meant to write this last week, but this and that got in the way. Hm. I'm leaving for Toronto tomorrow. 24 hours from now (7:38 am) I will either be sitting on the tarmac waiting to leave or I'll be in the air crossing the Alberta-Saskatchewan border.
There are several ways in which you can enhance or develop existing creatures into 'deeper' or more threatening monsters. I thought it might be helpful to review them and consider some of the effects of each, as a helpful overview in making new monsters and appreciating the aspects of old ones.
Let me pause first and explain that monsters are not targets. They can be used as such, and many a campaign will see them as nothing but video game pigeons to stand up and be shot down - hacked down, melted down - in order to measure the luck or success of the players. The stand-em-up, shoot-em-down process is the principle reason why combat is seen as boring in most campaigns, as little or no investment is made in making monsters squeal, bark, shout orders at one another or otherwise interact with the party during melee.
I don't know if the reader has considered this. Envision the players as participants in a football scrimmage, speaking the same language (as they would in my world) or not. Have one of the goblins on the other 'team' point at a player and shout, "You! You're going down, you bastard!" while another cries, "Your people killed my mother!" Another cries "GELF!" at the elf while another screams "Your mother was a human fucker" at the half-orc and the stage is very quickly set.
Most combats - even my own, when I'm hard-pressed for time - are played out in a sort of role-play cone of silence. I have found that it is always more effective to have creatures snort, yelp, bawl, bellow in pain, clamor, whoop with pleasure (try this after a character drops unconscious or dead) or otherwise make a racket.
The more personality, the better - so in considering how to create or enhance monsters, the reader should definitely keep this in mind. Statistics are far less interesting or meaningful to the players than purpose and behaviour.
Make monsters bigger. This is the most obvious and most employed tactic. No matter what it is, making it bigger makes it more interesting. All the giant forms of earth creatures employ this, while it is equally as much fun to double or triple the size of something already made huge. A 7 hit die giant crocodile is already 30 feet long - why not a 21 hit die titanic crocodile too long to be seen all at once?
I must admit I've never had any trouble with this sort of creature. Bigger is always better, even if the bigger expands to incomprehensible size. Players are amazed and effectively terrified by mass - just so long as there's still a chance of retreating or hiding when things reach dimensions of Paul Bunyan and Babe the Blue Ox. I once had my party (with all their henchmen and followers in tow, a total of some 60 people) combat a roc with more than 1,000 hit points, so big that its most effective combat was landing, beating its wings and rolling about on the ground (1d20 incidental damage to everything).
If there is a limitation to this, it's that not everything can be BIG. It gets tiresome.
Make monsters smarter. This only works if you're the sort of DM that runs your smarter monsters differently than those that are dumber. I have seen many a campaign where a so-called 'genius' beholder uses a frontal attack like an orc running forward up a hallway. For me, any creature above extraordinary intelligence will interact with the party in only one of two ways. They may want to talk, in which case they're more likely to encourage the party towards an action rather than threaten them. If they wish to fight the party, they're more likely to do it through minions - sending wave after wave against the party until these run out. Whereupon the intelligent monster would make itself scarce. The only way that a party would ever likely encounter a beholder in my world would be if it were cornered.
I've had a party fight a beholder once. But I was very young and I never did think I did a good job of it.
I would hate to be a party fighting one in my world now. As I have grown smarter, so have my genius-level monsters.
Of course, I've tried creating a scale of tactical/weaponry advantages for very intelligent/high intelligent monsters, but I haven't quite incorporated that into my mindset. It's something that will come with time.
Smart monsters make for more role-play outside the normal character-vs-authority figure or character-vs-criminal dynamics. Monsters should have other motivations, even irrational motivations, that should serve to make the game more interesting. They want things other people don't want or feel offended by things that would never bother a human or demi-human. Both will need a DM to stretch their imagination.
Make a lot of monsters. If the individual monster isn't particularly dangerous, that's no problem - make sure they exist in great numbers. A few days ago I introduced the 'purple frog,' an ugly little beast that's only three inches long and therefore no particular threat. Naturally, the first inclination is to make it bigger, even if it is only goblin-sized; or alternately to give it an intelligence, since talking to a little ugly frog that's 3 inches long probably would make a party stop and think.
Alternately, we could make thousands of them.
Ever experienced an insect invasion? I remember a two-week tent caterpillar infestation when I was young - several communities in northern Alberta had one this year. Here's a nice short video for viewing. The experience is somewhat less than appealing, let me say. The sound of rain that can be heard is actually caterpillar droppings falling on leaves, while the crunching of caterpillars under car tires reaches a decibel level that can be heard fifty meters away. All one can do is escape - and still the occasional caterpillar can be found crawling over the carpet, on the window, poking through the garbage and so on. So much fun!
Imagine millions of completely harmless, completely normal purple frogs scampering underfoot continuously for two weeks of game time. Imagine a party unable to get free of them, unable to sleep at night, unable to step without crushing them, vomiting and taking damage from the smell, possibly incurring a disease, having to fight a maddened beast driven crazy by the infestation while the party slips and slides on purple frog guts. There are all sorts of places.
Of course, everyone knows to increase the number of orcs or kobalds as a party increases in levels - but true infestations hamper everybody.
More to come. I have other ideas, of course - above are simply the most common three things. I don't, however, have the time to continue. I promise to do so, along these lines, when I return. This is enough for today.