Tolkien explained the development of beorn “man” as a blending of primitive ᴹ✶besnō “man” and ᴹ✶berō “valiant man, warrior” > ber(n)ō (Ety/BER, BES). Since there is no clear reason why a nasal would be added to ᴹ✶berō, the simplest explanation is that ᴹ✶besnō > bernō. Since it is known that [z] became [r] in Danian (PE19/28), this can easily be the result of [s] voicing to [z] before voiced nasals [m] or [n]. This possible development was suggested by Helge Fauskanger (AL-Nandorin/beorn).

See Dan. beorn for further discussion.