I'm fairly certain Nightmare Creatures is D3D through and through, so no Glide or SGI support. If this is the case, it is likely that all the pre-launch graphic profiles do is activate specific parameters or command-line switches that enable or disable features, use specific, optimized methods for certain effects etc to ensure that the game is running to the best capabilities of each card.
Assuming all of the above, then there's no reason the PowerVR or any profile select-able in the game launcher would refuse to run on the G400 or any D3D card.

Haki26 wrote on 2023-03-02, 09:52:

A few months ago I tested a matrox g400 with several games. Among the various games tested, there was also Nightmare Creatures. I realized that in the graphic settings of the game, you could also choose powervr. I tried to select it and doing it gave me no error. Both starting from the game menu and during the game there were no graphics or performance problems. Everything worked great, nice graphics and great performance, no bugs or artifacts or anything. Could you tell me the reason? The g400 is not Powervr, right? How is it possible that it didn't give errors?

Nightmare Creatures is PowerVR-ready game. Its not true SGL PowerVR game, but Direct3D with max. PowerVR compatibility.
So G400 just runs it in D3D mode. If you have real PowerVR card you can run it with it selecting D3D accelerator by 3DCC utility.

The rendering method of the NEC PowerVR uses a very different approach than what was commonly used at that time. Everything step in the chain was thought to improve the rendering process. New techniques have been implemented for that goal. The chip drivers for Windows making the process transparent for the communication with the common graphics APIs (as it is the case for every hardware anyway).
If the game propose a PowerVR option, it is probably because it include a implementation of the native PowerVR API. So maybe the game is trying to load its PowerVR interface which fail to load because there is no compatible DLL, then revert back to the first compatible API found (DirectX 3 or 5 I would say). Explaining why there is no error message. If the selection is kept to "PowerVR" is probably because of bad programming. The game engine doing things that the UI is not able to manage. Of course, it's all assumption here but quite plausible.

Thanks all for the replies, now I have a clearer idea of ​​what could have happened.
Now I also remembered that by setting Powervr I got a much clearer image than direct3D and maybe (I'm not sure about this) it also had more "rough" graphics.
I don't remember though if there were differences between Powervr and software rendering modes. I would like to test it again, but unfortunately I no longer have the possibility