The Nougat beta builds for the Galaxy S7 already have a selector for Games and High-Performance modes, but Samsung looks like it's working on taking this to the next level.
The Korean company has filed for protection of the brand "Beast Mode" in the European Union. The trademark will be used in relation to smartphone software, according to the filing, so it's pretty clear that we should expect Beast Mode to be in the Galaxy S8 when it becomes official in April.
Unfortunately there's no exact way to tell what Beast Mode actually is, since there's no detailed description to go with the trademark application. Judging by how it sounds, though, it has to be something to do with performance, so perhaps the a fore mentioned Games and High-Performance modes seen in the S7's software will have a third sibling in the S8. Since VR is the latest buzzword (don't call it a fad) in the mobile world these days, Beast Mode may be all about that.
The Galaxy S8 will be a beast in its own right, if we go by past rumors. The phone should sport Qualcomm's Snapdragon 835 SoC, maybe even 8GB of RAM and a fingerprint sensor on the back, stereo speakers, Bluetooth 5.0, but no 3.5mm headset jack. It could also be more expensive than its predecessor.