First, I suggest this steam guide by Gardivanyth, and Leinfors' (bless him) official Ultimate Guide To Content Creation And Use For The Terraria Workshop[forums.terraria.org]

1. 95% of item IDs can be easily found (ctrl+F) here: Data IDs[terraria.wiki.gg]

This makes finding IDs quite simple

2. True Goldfish form is just a butchered use of the torso and legs from the Werewolf form. To be frank, the arm (fin) isn't really displaced "correctly" in first place: I tried that and it offset the arms very badly, not surprising because the arm is coded to pivot from a specific point. I just left it as is, passable at surface-level.

3. Yes, the ID's for Werewolf and Merfolk form I could not find on the wiki despite best efforts, so with just founded first image file in for each part: armor (torso), leg, and head; then clicked right arrow key until I saw werewolf (merfolk file is right beside in each category).
(You should have extracted terraria files for texture pack convenience, if not, see Gardivanyth's steam guide on "unpacking the assets.")

I don't have tips other than the Merfolk form sprites are the Werewolf form +1 (plus one). Using either Werewolf or Merfolk form in texture pack, just know that it hides all parts of player (which is why it was used for goldfish).

The Moon Shell (Moon Charm / Neptune's' Shell) IDs for anyone's convenience:
Werewolf form

• Armor_Head_38
  
• Armor_21
  
• Armor_Legs_20

Merfolk form

• Armor_Head_39
  
• Armor_22
  
• Armor_Legs_21

4. I haven't done anything that would require you to cite or reference me. Unless you use a texture pack of mine, but that would be for other's convenience, not me because I don't really mind honestly.

Hope that helps, let me know if I missed something or needed elaboration on something else.

thanks a lot for making it so thorough!
with this I should be able to make the custom sprites and make my own re-texture!
thanks again!

Oh hey, i'm back! well, I went and finally took a spin at making it, its been tons of fun.

But I remember you mentioned something about issues involving not being able to displace the arm and it being finnicky due to the code determining the rotation point, and henceforth using a compromise.

I have arrived with something to maybe contribute and help!

since the "rotation" point is determined by code. you can offset the arm, and then, put a sacrificial pixel at the spot, to "trick" the system into displacing the arm. and since terraria works with alpha layers, I was able to displace my arm, with a single pixel that was at 1% opacity. (since terraria works at 200% scale, I was technically also able to make the pixel just one fourth of the size it occupies normally to really send it home.)

I even tried using the martian dye to see if that one 1% opacity pixel would get highlighted, but I saw nothing!


Now, determining the rotation point to put this seemingly invisible pixel was pure trial and error, so I dont have any accurate measurements or the know how. hopefully this might be useful to you

Haha, that's awesome! I never bothered to experiment; first attempt was offset, didn't work, didn't imagine a solution due to code, and moved on. Quite foolish of me, clearly my imagination failed me.
I tried using transparent pixels only one time, but that was to try to make an icon outline show up for the map but not visually in game. I have this pack I am planned that would have offset arms, so I am psyched to give your solution a try when I hopefully get to that. Thanks for sharing this, much appreciated!

I'm very glad this little thing I found might actually be of use to you!

also, with one more report. no clue if its 100% consistent on Tmod loader. but I have had cases where the werewolf transformation actually does not hide the head model. just a heads up.
lowkey im pretty sure its one of my millions of mods so take that note with a huge grain of salt.

Unless tmodloader specifically modifies the Werewolf transform, yeah I imagine it's a select mod altering that; in any case, I appreciate the information. <o