How do you get more of this monster?
Some of these overlap a bit, and edge cases can fit into multiple areas - do life-draining undead reproduce by infection or by transformation?
Again, choose one or just roll a d6!
1) Sexual Reproduction - The monsters simply breed to reproduce. Who or what they reproduce with matters, too. If they reproduce "normally," they'll mate with others of their own kind and produce eggs or young. If they can reproduce with other species, they're more likely to be interested in slaves or captives (or even mates acquired through charm spells, negotiation, or wooing). If they must reproduce with other species (say, for one-sex creatures produced from mating with another species), they are likely to be even more driven to contact and mate-getting. A species that can mate with anything else will have different attitudes than one that can only mate with a specific other species.
How common their mating partners are will affect their attitude towards them. If females are common, they might not be held as very valuable or as disposable. If males are common, how many die before mating isn't very important; they're replaceable.
2) Infection - The monsters reproduce by infection. This can be literal infection - AD&D's "lycanthropy as a disease" approach or zombies-as-diseased movies. Or it can be a figurative infection - victims slain by X become X, like many kinds of undead, or anyone hurt by a werewolf becomes one. Vampires are yet another one of these - bite a certain number of times and poof, you're a dracula too.
Reproduction-by-murder is just another form of infection; it just requires dead victims instead of still-living victims.
3) Splitting - the monster just gets to a certain size and then buds off, splits off, or otherwise produces young. Jellies and puddings often split like modern starfish, where a lopped-off bit becomes a new starfish. If they can only reproduce by being split, "big enough" creatures might purposefully attack in order to get chopped up so they can reproduce. Jellies might jump off cliffs like proverbial lemmings in order to break up and propagate the species.
This may explain the troll's independent lopped-off limbs and propensity for violence. They just need a arm or two lopped off to get a few kids!
4) Transformation - like infection, but the monster literally transforms its living victims into more of itself. The classic green slime does this. Although it's not "reproduction," liches also result from a self-transformation. So do ring-wraiths, albeit with lots of trickery ("Here, put on this magic ring. I made one for you and your eight friends.") This also suits one-at-a-time monsters can come back if the curse on their Magic Stuff gets the next person. ("I'll just pop this eye into my socket. Who was Vecna anyway?" or "I found these magic knives . . .")
5) Host Carriers - Similar to #2, infection, the monster reproduces by laying eggs into a victim. These usually parasitic eggs use the victim as an incubator and then food source. Reproduction requires victims, and those victims are carriers until they become food. This type of reproduction differs from infection in that typically the egg-layer can't just willy-nilly infect everything it touches. The more infrequently they can lay eggs, the more ferociously they will protect the host afterward. After all, you're carrying their young, and you're the kid's food source . . .
6) Creation - the race is created. Dwimmermount's dwarves are a great example of this - they carve their own sons (and occasionally end up with a gnome!) Reproduction may be race-specific - only X can create another X. Or it may be possible for a sufficiently knowledgeable member of another race to create it. Reproduction by cloning (magical or otherwise) is similar - if members of the race can do it, only raw materials might hold them back. Sometimes specialized external knowledge is needed, and the monster can't do that itself - golems are a good example of this.
Reproduction by summoning (get more from somewhere else) or by spontaneous creation are similar. All you need is the appropriate "raw materials" - power, corpses, rotting fruit, sacrificial victims, patches of darkness - and you'll get more of the monster. Summoning may just be a dodge, however - how do they reproduce where they were, if they reproduce at all?
For the rest of the series:
Monster Ecology: What Does This Monster Eat?
Monster Ecology: Where Do They Come From