There are several different styles of berserker practice. This is true both of the ancient ways and the modern. It seems that while different berserks tend to have the experience of becoming different animals or spirits members of each type tend to have certain things in common with each other.

Ulfhednar

An ulfhedinn (that's the singular) is a berserk who experiences becoming a wolf in the berserker trance. Or becomes possessed by a wolf spirit. Or whose internal nature is partially that of a wolf. Or who hallucinates becoming a wolf. Or however you want to look at it. Ulfhedinn means "wolf-coat". Ulfhednar tend to be very active, and move about constantly, often shifting guard and stance, constantly altering them, circling the opponent, bobbing the head. They tend to be very intellectual fighters. Some excel at ring control and precise movements, others utilize deception, tricks, and head games. The constantly moving ulfhedinn is very difficult to hit, because it is not so easy to set up a hit on an opponent who won't keep still and who never uses the same defense for long. Speed is often emphasized in ulfhedinn style, and accuracy. I have fought one ulfhedinn who is impossible to get a square hit on. Though much smaller than me he is so fast and such a great judge of distance that no matter how hard I hit I find that he has slipped his head a little bit, or twisted his body, so that the main force of the blow is lost. Ulfhednar seem to be the most common sort of berserk in modern times.

Bjornsterks

Bjornsterk means "bear strength". This is the term we use for those berserks who experience becoming a bear in the berserker trance. The historical term for them is "berserkers", because "berserk" didn't just mean "bare of sark" it also meant "bear shirt". But as these similar words were too confusing, as one was a general term and the other a specific one, we changed the specific term. Bjornsterks tend to emphasize strength, and many are grapplers. Fancy footwork or excessive motion doesn't seem to be common in the bjornsterk style. I know one who basically just uses his great resistance to harm and his iron helmet of a skull to walk in a straight line into his opponent's guard, shrug off the shots he takes, and throw his opponent to the ground. Bjornsterks seem to have been the most common type of berserk in ancient times.

Chatti

Chatti means "cats" in Latin, and was used by Tacitus and other scholars to describe certain barbarian warriors. They may or may not have been berserks, the references aren't clear. It is nonetheless the term some modern berserks have taken to describe those who experience becoming a large predatory cat in the berserker trance. The chatti I've known haven't been very experienced martial artists, and so I find it hard to really describe them. It does seem to be a more intellectual style.

Vanawods

Another modern term, it refers to those berserks who do not experience anything like becoming an animal, who instead just go through a derangement of the senses like a quick trip through madness, and the term itself does refer to insanity. The vanawod fighters I've known seem to emphasize aggression.

Spirit Horses

This term refers to those who have the experience of becoming possessed by a non-animal spirit, usually a more human one. It comes from the use of the term "horse" in some traditions to describe those people who become possessed by the gods and spirits. Some spirit horses have the experience of becoming possessed by ancient dead heroes, others of being possessed by gods of battle. I have never met any of these types. I have met two who had the experience of becoming demons in the berserker trance. Both fought with great aggression, and seemed to emphasize driving themselves ever deeper into the trance, so that pain tolerance became incredibly high, so that they can get close enough to let the aggression loose. Neither seemed to be that concerned with fancy moves or even defense.

Svipals

Svipal means "changeable", and this term has been adopted by some modern berserks who have the experience of becoming many different animals or spirits. They seem to fight with the style of whatever animal or spirit they are at the time. This can make them very hard to anticipate, though the experience can be overwhelming and distracting for the svipal berserk.

I have also heard tell of boar, bull, coyote, and other styles, but have never met any.