In short answer, the latter might be more likely in my understanding (as well as in the light of the current academic consensus) if such a tradition was really practiced in Viking Age Scandinavia, but all of these are of highly hypothetical nature.
While some scholars (mainly archaeologists like Lotte Hedeager and Neil Price) try to associate ulfheðnar (plural forms, < ulfheðinn in singular form in Old Norse, means "wolfskin clad warriors (?)") with the iconography of dancing figures with weapons on their hands found in Iron Age Scandinavia like Torslunda plate (linked to the picture uploaded to wikipedia), we don't have much written evidence either to confirm or to refute this hypothesis with ease.
I recently wrote two posts on the extant source and its interpretation on Viking Age berserkers (berserkir in Old Norse), with successive discussions with /u/itsallfolklore and his discussion with /u/textandtrowel:
+++
While I haven't seen the film in person, The official trailer of the Northman (from 01:49 onward, especially around 02:04-05) clearly shows that it is apparently heavily inspired by the somewhat popularized Hedeager-Price line of interpretation, a kind of (initiation) ritual practiced by the "outlawed" pack of warriors.
Dale regards so-called berserk fury primarily as psychological one (Dale 2021: 52), but it is almost impossible to distinguish whether the crazed warrior actually possessed by the animal spirit or only got some animalistic traits based on the extant written evidence.
The key text of berserk fury is actually late, Ynglinga saga written in the early 13th century, allegedly 2 centuries after the official conversion:
'......his (Óðinn's) men went without mail and were as wild as dogs or wolves, biting their shields, being as strong as bears or bulls. They killed the people, but neither fire nor iron took effect on them. That is called berserk fury.' (Snorri Sturluson, Ynglinga saga, Chap. 6, in Heimskringla. The translation is taken from: [Finlay & Faulkes (trans.) 2011: 10]).
To what extent the magic practices of Óðinn (Old Norse spelling of Odin) and his follower described in Ynglinga saga, especially with alleged "shamanistic" traits, reflect the actual beliefs in pre-Christian Scandinavia has also been heavily disputed - since it was another source of inspiration for medieval Scandinavians, namely the ritual practices of the Sámi people (Cf. Tolley 1996). To give an (possibly extreme) example, Lassen's monograph, the latest and most detailed study on Óðinn's representation in medieval written evidence, states that:
"The passage of Odin's outer-body journey appears to have been inspired by the shamanism of the Sámi and perhaps by the descriptions of Greco-Roman gods and is therefore not usable as a model with which to understand Odin as a shaman, and therefore hardly of Odin as a 'shaman god' (Lassen 2022: 158)."
Thus, the interpretation of the cited text above is difficult, or almost inevitably inconclusive in accordance with the current standard of source criticism.
On the other hand, ulfheðnar with the idea of outlaw pack in the woods in fact has a clear model in Old Norse literature - Sigmundr and his son, Sinfjǫtli (half-brother of famous Sigurðr Fáfnisbani (the dragon-slayer)). In Völsunga saga, they once put (apparently magical) wolf skin on, then changed into wolf shape.
Sigmund and Sinfjǫtli, especially latter, had apparently been quite popular in pre-Christian Scandinavia, even more than the latter's brother Sigurðr. Sinfjötli appears in several eddic poetry, and the anonymous skaldic poem Eiríksmál (middle of the 10th century) put them in Valhalla as representatives of legendary champions of Óðinn, but not Sigurðr:
"Sigmundr and Sinfjǫtli, rise quickly and go to meet the prince. Invite [him] in, if it is Eiríkr; it is he I [Óðinn] am expecting now (Eiríksmál, Stanza 5. Translation is taken from the official site of Skaldic Poetry of the Scandinavian Middle Ages)."
Eddic poem Helgakviða Hundingsbana I (First Lay of Helgi Hundingsbane) certainly has suggestive mentions on Sinfjǫtli's childhood with the association of the wolf (Stanzas around 35-41), but the allusion is not so explicit to describe the outlaw pack of wolf-clad warrior appeared in the Northman, as narrated in later Völsunga saga.
I'm also not sure about the current academic consensus on to what extent this alleged Sinfjǫtli's association with the wolf should be regarded as personal or common among other ulfheðnar in general, though.
Anyway, possibly due to his linage in later tradition (born from the incestuous relationship between Sigmundr and his sister Signy (who disguised not as her under magic, though)) or Wagner's handling of him in the Rings (merged with Sigurðr-Siegfried and erased him completely), Sinfjötli have attracted not due attention both from scholars as well as from popular interest in Old Norse literature and the Vikings, I suppose.
If you find the revenge tale narrated in the film Northman very interesting, I'd recommend you to check Völsunga saga's translation on the fugitive and training life of Sigmundr and Sinfjötli in the woods.
References:
+++
(Edited): adds the singular form ulfheðinn for the sake of better search-ability.
