1->To become a mob you have to type in chat "/ function dis:disguise / <mob>" or "/trigger disguise set <mob id>"
2->To remove the disguise you have to type in chat "/ function dis:undisguise" or "/trigger undisguise"
3->To modify the nbt of the mob you have to type in chat "/ function dis:options/<nbt>" or "/trigger options set <option id>" (down below there is an explaination of this part)
4->To disguise/modify another player's nbt you have to type in chat "/execute as <player> at @s run <function (without the slash)> (only for opped players)

1st step->Type in the search bar "%appdata%" and then enter the ".minecraft" folder.

2nd step->Then enter the "saves" folder and then the folder of the world that interests you.

3rd step->Once this is done, go into the "datapacks" folder and then into my datapack folder.

4th step->Now follow this path: Disguise_1.17.1_v2.5> data> dis> functions> disguise> custom_mobs".

5th->Now choose a template function. You can import a total of 50 custom mobs into the datapack. Just open an empty template and where it says "<insert the HP of the mob>" you have to put the exact number of HP(e.g: the player has 10 hearts -> 20 HP) and remove the little arrows("<", ">"). Where it says "<insert the mob ID here>", you have to put the ID of that mob(the ID you use to spawn it with the /summon command), and then remove the little arrows("<", ">"). And finally it's time for the abilities. You will find a list of all the available abilities. You can keep the line and removing the words between the arrows ("<",">") if that ability matches the mob, if not, erase the line completely. If you can't find the function while typing it in the chat, it means that something is wrong in the function.

set_ai:
-free=The mob will be able to behave naturally (for example, pillagers will be able to attack villagers).
-no_ai=The mob will simply act as a disguise.

set_baby:
-false=The mob (if possible) will become bigger (for example: mini_zombie->zombie).
-true=The mob (if possible) will become smaller (for example: zombie->mini-zombie).

set_burning:
-false=The mob will stop burning.
-true=The mob will start burning.

set_carpet(llama&trader_llama only):
-The llama will wear a carpet over it.

set_charged(creeper only):
-false=The creeper will become normal(its explosion will become smaller).
-true=The creeper will become charged(its explosion will become bigger).

set_chest:
-false=The mob will no longer carry the chests.
-true=The mob(if possible) will bring chests.

set_color(sheeps only):
-Depending of the color the sheep will take on that color.

set_grumm:
-false=The mob will return normal.
-true=The mob will be renamed "Grumm"(and it will be turned upside down).

set_horse_armor(horses only):
-Depending of the chosen armor the horse will wear that armor.

set_ignited(creepers only):
-false=The creeper will return normal.
-true=The creeper will activate.

set_jeb(sheeps only):
-false=The sheep will return normal.
-true=The sheep will be renamed "jeb_" (and it will become rainbow colored.)

set_llama_color(llama&trader_llama only):
-Depending of the color the llama will take on that color.

set_saddle:
-false=The mob will return normal
-true=The mob(if possible) will wear the saddle.

set_sheared(sheeps only):
-false=The sheep will return normal.
-true=The sheep will be shorn.

set_silent:
-false=The mob will go back to making its noises.
-true=The mob will stop making its noises.

set_axolotl_variant:
-Depending of the color the axolotl will take on that color.

set_screaming:
-false=The goat will not be a Screaming Goat.
-true=The goat will be a Screaming Goat.