Let's be honest, homebrewing enemies from scratch is time consuming. It can be incredibly fun, if you're into that kind of thing, but balancing enemies without playtesting them first is incredibly difficult. But, on the flip side, so often it seems impossible to find an enemy that fits despite the hundreds available in source books. My personal struggle is always that I want my big bad bosses to be conscious, talking, magic wielding creatures; yet most of those are either very high CRs or they're dragons. And I love dragons, but there's more depths to plumb. Plus, what if you want to have a ruler or other kind of humanoid be a bad guy? Taking down a king should be a difficult challenge. The solution that I have found to this and many other problems is simple. Find an enemy with the right CR and abilities that are close to what you're looking for, and reskin them. Coming up with new enemies and bad guys is pretty much half my job here at Only Crits, so listen up and you'll probably be able to cut me out of your life altogether.
What to Keep
These are the things that you should be looking for in an enemy you want to reskin. These are parts that make an enemy interesting, and are difficult to come up with on your own. Now these may vary from person to person, for instance when I started out I struggled with balancing HP/AC, so those were the parts that I liked basing my reskins off of. But, as I got better at recognizing what made a monster fun to fight (and when I actually read the source books and found pg. 274 of the DMG) my view switched, and I started keeping unique attacks and abilities. So, here are my recommendations on what to keep when reskinning a creature.
1. Traits: These often factor into fights more than you'd expect, and help color an enemy. If you start eliminating traits, sometimes you'll end up with a bit of a bland bruiser-type enemy.
2. Attacks: Keeping attack bonuses and damage goes a long way to keeping the balance of an encounter accurate. If you want to play around with them, change the type of damage, the range, or the weapon.
3. Reactions: Same as traits and attacks, these are great for balancing and also tend to be great color for enemies. Reactions are so fun, that sometimes you might want to take an attack (one that you might not actually end up using on your monster's combat turn) and make it a reaction instead.
Things to Edit
These are where the fun part of reskinning happens. It's where the monster in the source books stops being something that you're simply using, and becomes an inhabitant of your specific game. Again, these will vary a little bit, and obviously these recommendations are nothing more than that. If you want to play around with changing attacks and leave these categories untouched, absolutely go for it.
1. Damage Resistances/Immunities/Vulnerabilities: The way that a monster reacts to being damaged is vital. Especially if everyone in your party is rocking Fireball and Firebolt, suddenly coming across an enemy that has fire immunity makes for absolutely wonderful fun.
2. Speed: May seem like not a big deal, but when you have a rogue that can disengage and get out of range, or a wizard that never gets into range, having an enemy that can suddenly close distance can be huge. Or, conversely, if you've edited an enemy to have stronger attacks or higher damage, then maybe they struggle to actually get within melee range to make those massive attacks.
3. Appearance: Purely for roleplay and making sure that your new, reskinned enemy fits perfectly into whatever cave, castle or dungeon you put them in. Sometimes appearance will mean more, sometimes less.
Now, Try the Opposite
Just for the fun of it, flip those two categories that I've just listed. Keep an enemy looking like they always have, but have them deal way more damage and react entirely differently. Players getting too meta? Understanding how certain enemies behave or how much damage they can expect to take? Change it up. These categories highlighted have worked for me, but you also don't want to fall into too much of a rhythm when you reskin or you'll become predictable. As always lean into what's fun, both for you in the preparation and your table in the game, and you can't go wrong.
A Note on AC, HP and changing CR
To finish I want to touch on the categories that I didn't mention, as well as if you want to increase or decrease a monster's CR. I think when it comes to changing CR I might do a separate blog post, since it is a more involved process. That being said, playing with a creature's AC and HP are good ways to accommodate reskins, but I didn't want to include them as whole categories because if you're doing nothing but playing with HP and AC the enemy won't really feel any different to play. If you want the fight to last a little longer buff them both, but be aware they're going to have a better chance of wiping PCs. If you want to fit the health and armor more accurately (say you make an enemy appear huge instead of medium, but without any armor) then you can buff one and drop the other. This is one of the tougher parts of editing enemies because it's just hard to know if you've done enough of one or another. If you drop the AC by 1 it may seem insignificant, but it could mean that every attack hits instead of every second. All this to say, play around with AC and HP because it can be fun and can have some changes to fights, but try not to get too caught up and worried about it. Focus on the fun stuff.