Hexing is the process of modding the Petz game to create new breeds, or other items such as clothing or toys. The name is derived from the hex editors that were originally used to modify the game files. Although these days we have much better tools to work with, the name stuck.
The two main hexing tools in use nowadays are:
Petz Workshop - a visual editor
If you're interested in learning to hex, I suggest that you take the time to learn how the LNZ files (the files inside a breedfile that dictate what the pet looks like) work. The numbers might look intimidating at first, but you will gain a better understanding of what you're doing and be able to pull off some interesting tricks. I personally hex using LNZ Pro for most of my work, though I do like Petz Workshop for visualisation of fiddly bits and placing paintballz.
The best resource for learning LNZ files is Carolyn's LNZ Breakdown, which details all the parts of the LNZ file and what they do. It's an invaluable resource and you can also find it in the LNZ Pro helpfiles so you'll always have it on you.
Below are some tutorials I've written (or plan to write):
(Not all of these are converted to .gmi. Everything I've written so far is on http:
https://lilaclynx.net/petz/hexing.html
Bear with me...)
A tour of the breedfile in LNZ Pro
Viewing your breed as an adult in the Adoption Center
Making a basic variation set
Using linked variations
Variations in addballz
Making "magically appearing" surprise features using addgroups
Making growable features (horns, wings etc)
Avoiding the double tail bug with addball tails
How ears work in catz and dogz
Addballz per breed
Swapping breedfile personalities
Beyond breedfiles: exploring the resource.dll files
Assorted notes and discoveries from delving into the unknown depths of the game's workings. Not tutorials, but may be of interest to some.
SCP/Personality Research Notes