Before I knew it was supposed to be hard to do it, I converted adventures from one RPG system to another on a regular basis. It started innocently enough. I'd play in someone's Tunnels & Trolls campaign, and then go home and run the same story as a D&D adventure for my friends. It never occurred to me that adventures written for one system weren't considered compatible with another system.
Unsurprisingly, published adventures for D&D do tend to be pretty specific to a version of the game. That's both a great feature and a an annoyance, depending on the use case. When I ran players straight through the Ghosts of Saltmarsh adventures, it was great to have all the right stat blocks in exactly the order they were encountered. On the other hand, I usually use modules out of Yawning Portal out of published order, and so I end up swapping out or customizing most monster encounters.
By far the hardest part of adventure conversion, for me, is finding the right monster swap. Most of the time, I don't care whether the player characters encounter a Aboleth or a Yeti, I just don't want them to encounter a Kuo-Toa while roaming the deserts of Zakhara. In other words, it's not the specific monster I care about, it's the appropriateness of the monster in the context of the story.
It's an easy fix when a party is meant to be attacked by, say, a CR 1 goblin boss, but my players are all level 8. No problem, I throw a CR 6 hobgoblin warlord and a few CR ½ hobgoblins at them instead. Different monsters, different lore, but still goblinoids and probably suited well enough for the environment.
When an adventure calls for something less generic, though, it's a little tougher. What's the drop-in replacement for a Hook Horror? You wouldn't want to just select any monster, you'd want something specific to the Underdark. What's the scaled-up version of a Lamia? And so on.
In short, how do you find the different CR monster to stand in for what's written in an adventure?
Well, it turns out that the 5e DMG has exactly the answer. Appendix B on page 302 has a list called Monster by environment, which categorizes monsters as arctic, coastal, desert, forest, grasslands, and so. The monsters are sorted within each category by CR, so you can find the environment of the encounter you're adjusting, locate an appropriate CR level for the average player level (APL), and grab your Monster Manual.
For the same information online, there's the Donjon monster database, which helpfully adds in monsters from Volo's Guide and Mordenkainen's Tome.
Of course, as you get into high levels of play, your options start to dwindle and you're just as likely to desperately thumb through the Monster by Challenge Rating section searching for anything above CR 12.
I wish all publications containing monsters had this kind of list. It's not uncommon for bestiaries to provide a CR list, but the environmental list is invaluable. In fact, I almost would go so far as to say that without an environmental listing, a bestiary is next to useless. The only saving grace is that you can flip through it casually and stumble on something, but a lot of times it feels like a crime of opportunity when it should feel like exciting arcane monster research.
Public domain illustration from Freesvg.org.