ElectricGod, I don't think that postin something like that here is a problem if they're just looking for answers. Now, if you're saying to literally have the brain or full mentality of a nonhuman animal (ie, a cat), then I understand why you woul find it weird that they post it in seriousness anywhere.
Mental shifting is shifting your brain to take on aspects or instincts of an animal, Or so people say. I personally think it's possible to take on personality traits of an animal, but I don't really know about this..?
Honestly it is possible but hardly. When people say that they usually mean that if you think of all the traits of animal you will slmehw change for that cause you thought about it enough and you now believe it. You can't really and your human for a reason, there are some however who are born with gifts that almost seem animal like. Via this change.
In Norse Paganism, there is a technique that is just as you described, called Berserkergang, in which the person mentally takes on animalistic characteristics through various methods including trance. The purpose of this originally was for fighting, but has other uses as well.
In other shaman practices, the shaman 'transform' into an animal, although this is often taken as shape-shifting, which is not possible , it is really the shaman embodying the spiritual aspects of the animal.
I haven't much experience with this, but there are plenty of practitioners on this site who practice Berserkergang, if you want more information you need to look no further than the 'Norse Paganism' section of the site, there Personified has written a couple great articles about the subject.