Perhaps a bit too similar to (or basically what you're going for with) the Smokebomb and Replace enemy, but a changeling enemy that pretends to be one (or more) of the party, mimicking their moves somewhat incorrectly. At the start of battle, they cast an AoE on the party that (reduces the accuracy of/make them invulnerable to) all but one of the Princesses (whomever they are mimicking) due to the others uncertainty, until the enemy is beat or the spell is broken somehow. Perhaps doing some naughty stuff to try and 'awaken' memories, or 'love' based variants of the normal attacks.

Depending on complexity, you could have an encounter where a Replace changeling replaces one of the Princesses at the same time as the enemy "Pretender", leaving the player to battle the real Princess (distinguishable by a subtle difference in the sprites/attacks used, and/or immunity to certain effects). It would effectively be the same encounter, just have a hidden status effect on both the enemy and the princess to determine which sprites are used, and the outcome when one of them is defeated.

Might have to have a trigger for when the Replacer realizes they've been had though, and they turn on the Princesses.

Or instead of being the princesses themselves, fake love interests (or Shining Armor in Cadance's case) that pop up out of nowhere and try to gaslight the Princesses into believing they've been made to forget them. Desperation leading them to try and reignite their love with true love's kiss (and more). Trying to prove they're genuine by 'revealing' their target Princess is into certain kinks, and forcing them into it. The target would have to be determined at the start of the fight, or during the first round.

Alternatively, a trio of Changelings pretending to be the Princesses. Being defeated by themselves, or even the sex slave variants of themselves, would be humiliating.

Floor/wall traps that are green cocoons. One princess is partially restrained and/or stripped, while the other two have to deal with a small ambush. Could have an enemy type that initiates the animation mid combat, similarly to the rope enemies.

For the Changelings as a whole, you could have a theme of heart shaped aphrodisiacs/pheromones. Make Alicorn physiology resistant to the stuff, but leaving them briefly stunned (in lore) and with dulled senses. This plays into the existing Smokebomb idea, and could be the basis for the Pretender's initial AoE.

Building on that, could have an enemy type that plays similar to the Drone Tech, using pheromones for a similar effect to hypnosis. Plus, for more recycling, they could force an aphrodisiac filled gas mask onto one of the princesses for a variation of the Hypno Helmet animation.

Probably more work, but given that you have to take out the Slimer from behind, could be fun to have a still image of one of the princesses sending his equipment into overdrive, making him waste his shot in the wrong direction (possibly with friendly fire). Or if actually taking him down from behind, adding a lewd still/animation if there is a Replacer in the party, potentially even alerting the Slimer and betraying the Party to start a fight.

I once had an idea about this duo. I don't know if it fits the theme you're looking for.

Basically, an enemy would set traps during the fight, a balloon with an evil smile would appear above his head. If one of the princesses attacks him (either directly or indirectly), the damage of the attack is cancelled and the princess is captured (I was thinking of keeping her in a state of bondage, for example). Then this enemy starts to focus his "damage" on the captured princess.

And then the second enemy would have an ability that redirects damage from an ally to himself, this character wouldn't do much damage but would be super tanky, and after using this ability the captured princess can't be rescued by anyone else. (She could still escape on her own). So the point is that the first enemy captures a princess and focuses his damage on her while the bodyguard protects him.

The bodyguard, in particular, could also be used in a more creative way, not just depending on the first enemy, but being the bodyguard of other enemies, increasing the variety of his usefulness.

Again, I don't know if it would fit into the concept you have in mind, but I'll leave the idea in case you want to use it later.

Hrm... I could see the latex pony being useful. Perhaps one of your party members got latexified or suited or something, and you've gotta pick em out of a line up. Pick right and you get em back. Probably still with a status effect or something. Pick wrong, and, welp, now you've got a fight!

... warning great wall o text due to stream of thought.

hmm changlings and a slime producing methiod. say quakehoof are you familiar with the story of the tar baby? if not the tar baby in question was a trap that resembled someone that when attacked ensared its victom truely insidus, the more you strugled the more you sank in and were ensnared further. anyway if the slime sprayer could utilize a similar tactic then it could be a good set-up for a gurrenteed ambush with wasted player turns... just please give these traps a "time to live" otherwise they could clog passageways and such on the map. would make people check the ambush target a bit more closely too to avoid reciving a surprise for their surprise.

on the changelings replace ability, could be a good opertunity for fakeouts and buggery. you only think they replaced a party member, when in reality they instead did it to an interactable/object nearby that when you get too close (3-4 tiles) to rams into the party with the speed of a train causing a surprise round and small ammounts of damage possably capturing outright if they are low hp... also could cause changlings in the party to take advantage of the mess to strike. could also cause quite the mess if one of the tar babies mentioned above was selected for this.

annother variation, your parties fine... but now the blighters in the inventory! you know one of your items are a fake but which one? that stack of 42 potions wasnt there before, right? or worse yet replacing your equipment, go into battle thinking everythings fine surprise celestia's clothing turns into living fetishwear! needless to say with vibrating plugs/tentacles and all the stimulation from the molestation i dont think she will be much help until you get them off her.

perhaps a bad idea, but how about changlings having a move styled after metronome from pokemon? that way if your REALLY lucky you can wipe to a boss move or end up with a status ailment you normaly wouldnt have... or blasted with your own spells too.

lastly on that latex pony, could use it as a placeholder for enemies that dont have graphics on the map yet, or do what annother game i seen did turn your party into drones and shuffle em that way you dont know whos who.