It depends on the license of the original model you were using.
Example … a Free Horse Model and a Free Human Model that states it can’t be used commercially or has to be used Open Source … then NO. You would be infringing their license and copyright.
If you bought the models or you were granted a commercial use or non-restricted license then you could chop and change as you see fit. However you might still hit issues if the license says no reverse engineering or if you try to sell it off as your own model (i.e. not in a game but as an asset).
I generally will buy assets commercially and modify them to my needs but they always stay in the game and I never release them as separate assets.
When in doubt … contact the author of the models in question directly … rather safe than sorry IMHO.