Gurlluas

So it turns out that just renaming the files actually works. I renamed it to rebreath.pck and there we go

@Gurlluas: cool underwater archaeological find indeed!

It does look like an Aquanaut, true. Man, kudos to you for the dedication to this game after all these decades.

So in the build there is a peculiar folder labelled TDXCOM_0
(Probably means Terror Deep)
This is dated two months before the build, our build dates to November 1994, while this sprite is from September 1994.
Here's the extracted graphics file. (It was also in the archive I put out earlier)
TDXCOM_0.rar
This appears to be an extremely early Aquanaut.
There is this early mockup, this is one of the earliest TFTD screenshots and clearly isn't actual gameplay but just sprites and tilesets put together to make a promotional screenshot. The Aquanauts there not only resemble the Ufopedia entry for Aquaplastic armor, but also are VERY similar to this unused sprite.

 
I went ahead and tried to put the pieces together to showcase it. As you can see it is very similar.


(I am sure I didn't put it together 100% correctly as I feel like a piece is missing, but you get the gist of it.)
This is almost certainly the very first Aquanaut they made, and while the final Aquanaut is based on the X-Com 1 Soldier sprite, these sprites were completely unique and different.

I have extracted all the graphics from these two builds. Leftovers from UFO look a bit odd since they are using the TFTD palette. 
This is truly a fascinating look into the development of TFTD.
The strings are also fascinating. Sadly no Shark at all. But here is an item list:
Dart Gun Dart Clip Jet Harpoon Harpoon Clip Gas Cannon GC-AP Bolts GC-HE Bolts GC-Phosphorous Bolts Hydro-Jet Cannon HJ-AP Ammo HJ-HE Ammo HJ-I Ammo Torpedo Launcher Small Torpedo Large Torpedo Phosphor Torpedo Gauss Pistol Gauss Rifle Heavy Gauss Magna-Blast Grenade Dye Grenade Particle Disturbance Grenade Magna-Pack Explosive Thermal Imager Medi-Kit Psi-Amp Thermal Tazer Sonic Cannon Cannon Power Clip Sonic-Blasta Rifle Blasta Power Clip Sonic Pistol Pistol Power Clip Disrupter Pulse Launcher Disrupter Ammo Thermal Shock Launcher Thermal Shock Bomb Sonic Pulser Zrbite Molecular Control Implant 1 Aquatoid Corpse Gill Man Corpse Lobster Man Corpse Tasoth Corpse Calcinite Corpse Deep One Corpse Bio-Drone Corpse Tentaculat Corpse Triscene Corpse Hallucinoid Corpse Xarquid Corpse  
A bunch of items have different names.
tftdtestdisk_extracted.7z

Quoting myself because I kinda have an answer. My take is that the test disks were used just to determine functionality of the maps and possibly AI and nothing more. The other devs, artists and programmers were all working on stuff "behind the scenes" so to speak but all of those resources didn't make it into the test disks... only a fraction did. And from more research I can say that weapon designs were happening weekly (if not daily) so we can only catch a glimpse or a snapshot in time of the development of them. Alien designs went through a number of revisions as well, but those all seemed to be internal. It makes sense really. I'd imagine the test disk creators just needed one alien to test functionality, the rest didn't need to be included. Because the designs were changing so often it would be hard to get each department to send you the newest resources and include them anyway.
 
- Zombie

I unzipped the files today and got the game running! It's only Aquatoids (they look the same), but a lot of other things are different. The game is apparently reusing UFO's geodata.dat file for the TFTD weapons so the aliens are ultra-vicious with 35 round mags for their sonic cannons. The standard aquanaut diving suit is yellow, but when killed, the corpse shows a blue suit. The aquanauts themselves stand with their legs far apart in an upside down V, which is really funny. All the weapons look mostly different. I was able to find the USO (looked similar to a Medium Scout in UFO, except a little smaller and with nothing inside) and kill the two Aquatoids. Just before I ended the mission I saw a muted-purple ball on the floor. One of my guys had enough TU to run over it and take a look - looked a little like Elerium did with roughly the same yellow crystals, but it had a band of red running through it. So my first impression is that this is GOLD!
 
Going to see if I can get it running under DosBox so that I can grab some screenshots for you guys to drool over.
 
Anyhow, I'll be adding more promotional item images starting tomorrow again, just wanted to give you all a heads-up on the test disks.
 
- Zombie

@Gurlluas: No problem. The sheer volume of gaming news tends to get a little hard to keep up with and I have been erroneously, but understandably, dubbed "Skynet" before already. 😜
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Find out what top-down action-RPG Titan Quest II's upcoming update shall bring with it here.

Yeah I am also using my Caspian for DW3 :3
 
I do hope we get proper new content soon though. Colonization was fun at first, but it got stale quickly due to the limitations.
Operations don't seem too interesting...Really hoping for something nice in the future.
An actual Guardian hybrid ship would be epic.
 

Personally I love the lore surrounding the Guardian Precursors, so I am kitted out in Guardian stuff.
I used to fly an exploration Alliance Chieftain, but I switched out to a Mandalay briefly, and then to a Caspian.
The Caspian has a lower jump range than the Mandy, but I can bring my Guardian SLF fighters.
 
Did my best to make it look like a Guardian hybrid ship :3
 
 

Huh this looks interesting, thank you for making me aware of it. I'll try it!

Hello, I am a lifelong X-Com fan and a prototype researcher.
 
So of course I was surprised to find that TFTD has a demo with a lot of early content.
I've made an article on TCRF documenting all the differences I could find. Check it out
https://tcrf.net/Proto:X-COM:_Terror_from_the_Deep