Reused Boss models/skeletons you've noticed.

[Jirah]

Oh yea most of the Sin eaters dyed white models with very few unique new models, Ampadapor statue, Ampdapor lion, most creatures just come back dyed white, im 90% sure the Sucubus skeleton was used yea.

OH and the Ruby Princess resembles Sophia quite a bit

[kaynide]

Just did a certain BLU required trial (level 50? 60?).

The boss, Sigfried, is Thordan with a gun.

[Nestama]

Speaking of Thordan, Exdeath uses the same skeleton as well (if he were ported from Dissidia NT to XIV, then his sword would be flying around a lot and his cape less obnoxious).

[Vyrerus]

A great many monster skeletons and gear skeletons are reused pretty much endlessly in XIV.

The Ananta are reskinned Lamias/Merrows, which are monsters taken from FFXI.

Turn 10 boss is a reskinned coeurl.
Turn 11 boss is a reskinned hydra. Shows up again in Twinning.
Living Liquid has been reused wholesale for S rank hunt in Lochs and also reskinned for Living Flame in Gubal HM.
The Man Sharks in Akademia Anyder reused the Hropken skeleton.
The uhh, sparkly dark spirit thing in the last pull of Akademia Anyder reuses the skeleton of the Lorelei from Sirensong Sea. Which is already a reuse of some other ghostly frame I think. Just hard going off memory alone atm.

The first boss of Amaurot is just a repainted sandworm.
Lamashtu in Eureka is a redo of Caliofisteri from Weeping City, and the trash mob towards the end of Amaurot is a reuse of Lamashtu with a palette swap.
The second boss of Mt. Gulg is a reuse of Ozma's cube form.
Last boss of Saint Mocianne's HM is an enlarged remodeled flan.

There are way way way way waaaaay more than this.

[Kisshu]

An easy one is any three-headed dog using the Cerberus skeleton.

Archaeotania, the special fate boss in The Tempest, uses the same skeleton as Ultima Weapon.

Two bosses in Eureka and the second boss from Anyder share skeleton with the second to last boss of Void Ark(tentacle fall enemies).

[TheMightyMollusk]

At this point it might be faster to come up with pre-SHB bosses that haven't been reused.

[Vyrerus]

At this point it might be faster to come up with pre-SHB bosses that haven't been reused.

*Slams my thinking cap on, hardcore*

I believe Shinryu's entirety is unique.

Phantom Train?

Guardian? Maybe?

Nael Deus Darnus, sort of. They've reused her and even reused the exact model for her Turn 9 appearance, but I guess technically as far as I can see there's nothing that's reused the skeleton? Probably a shot and a miss.

I don't think they've reused any of the Ivalice raid alliance bosses as hunt monsters yet, with the exception of Construct 8, but I haven't participated in S rank hunting since Stormblood.

Obviously they've never reused any of the Heavensward knight forms... I think.

It's hard to think of stuff that's never been reused or isn't an example of something that is a reuse of something older lol. I was about to say the second boss in The Burn, but no, that's just a reskinned Mag Roader.

Hypertuned Grynewaht I guess? sort of?

Bardam's Golem, though it's technically not engaged.

I guess the possessed Raen as second boss in Shisui reused Dun Scaith queen?

The Motherbit giant tomestone boss in Fractal Continuum HM wasn't a reuse, maybe?

Ivon Coeurlfist in Temple of the Fist, maybe?

Yojimbo, maybe?

Suzaku and Seiryu's human forms, I think?

[Kyrj]

What was it, Sohm Al hard. It wasn't the first but it stuck with me. The last boss is just a Diremite. Like one of the Diremites you see in Gridania. Just a plain ole Diremite and before that we fought some 15ft alligator fire dragon thing.

[Stepjam]

I don't mind this in general, but there are definitely a few major cases where you really would have figured they would have put in the extra time/effort. For me the biggest one was the major armoured angel enemy from the Shadowbringers trailer. It's an extremely cool looking design and fit ShB perfectly but when I found out it was actually just a reused boss from a hard mode dungeon (which I hadn't done prior to seeing the trailer) I was really disappointed.

I mean they didn't reuse that out of laziness, it was a specific choice. Same with the white lion from the cinematic. I forget what the exact lore reason was, but it's in the game.

[KageTokage]

They still haven't actually given an answer as to why the heck the Amdapori would've known what sin eaters look like, though Alphinaud does bring up the uncanny resemblance if you use him as a trust at Mount Gulg.