Resistance is futile. You will be assimilated.

[Savage Wombat]

Aboleths can create thrills, right?

I LOVE cheap thrills!

 

[Harzel]

The Yuan-ti have a transformed-from-human origin story, and still (reportedly) transform humans into Yuan-ti. In the lore-as-written they lack the hive-mind aspect, but a new group of them could arise with a hive-mind and an aggressive transformation agenda. Perhaps the embodiment of the collective consciousness (ala the Elder Brain) could be a ramped-up medusa, or just her head.

 

[Dannyalcatraz]

I LOVE cheap thrills!

Damn you! Damn you, Autocorrect!

 

[Swarmkeeper]

Now, with those factors in mind, what already existing monster or race would you recommend I use? (Bonus points if it already exists in 5E, but something from any edition will do. I don't want to homebrew something brand new for this, for various reasons.)

I'm willing to make a few minor changes/tweaks to the monster, but nothing that transforms them so completely that they're not recognizable.

In the Tome of Beasts there are Vine Lords who can create Tendril Puppets from infected humanoids.

 

[Fanaelialae]

I believe that Meenlocks (Volo's Guide) would also work, if you're looking for a fear based theme.