When morphing into a big cat i've noticed that its invisible when morphed into, same for all the mobs in this mod

Maybe caused by this:

Compatible with Hotfix 1.4.1 though.

Don't expect the Morph mod to work, at all. It is fundamentally incompatible with how I manage entity data (including HP/Attack values, skins, variants and scale) by default

And yes, you got it right, this issue was caused by unifying Bear and Big Cat species into a single entity instance, which I have no plans to roll back

That said, this issue seems pretty easy to fix, taking a look at the Morph's mod public repository, I have found the following. I will look into PRing support for UntamedWilds mobs in the near future, but I am kinda busy rn

Right.

-Compatible with Hotfix 1.4.1 and below.

Also the health is kinda buggy. The mountain lion should have around -28hp ish, but with 1.5.0 and above it has instead 40hp, i think this is the same for all the mobs but with different health from the original.
Maybe caused by the change i just mentioned.