ItemsAdder: I can't see the pet

Most likely you didn't import the file from Model Engine into your ItemsAdder resource pack, or partially or not correctly.

An easy fix is to take the folder assets into ./plugins/ModelEngine/resource pack/ and drop it where your "assets" folder is located in ItemsAdder (./plugins/ItemsAdder/data/resource_pack) or the other resource pack manager you use.

Merge the two "assets" folder (the one from ModelEngine into ItemsAdder).

In case there is any conflict, you can check that helpful tutorial to understand how to handle them.

If you have a conflict involving the item leather_horse_armor.json, overwrite the file from ItemsAdder if you want to prioritize the models.

However, note that conflicts that are handled by overwriting files will be problematic at some point, so you rather want to learn how to handle them as explain in that tutorial.