Can I install Android P Beta to my Galaxy S8?
It would be disastrous to do that !


As you mentioned it is only for Pixel device and flashing it on your device is same as asked here Can I install a ROM made for a different device?
Your device doesn't have treble 
support as mentioned here and you are wanting to flash a treble based ROM, which requires major changes to be done by developers


Unless you fancy an expensive paper weight , don't