[Turin]

Gurthang doesn't talk. Turin, or rather Tolkien through Turin, is projecting his other, conflicting personal torments: In his despair he justifies his imminent act (of suicide) by focusing on all the ill-things that he had done with ihis sword i.e. accidently killing Beleg, Brandir etc etc.

Not many animals do talk in the books. The Eagles of Manwe had been blessed by him and were a special case - their unusual size, for example. In the First Age they provide a valuable function for the Valar to keep an eye on Morgoth (Thorondor et al are an ominous sign to Morgoth that the Valar have not completely forsakened Beleriand).

Gwahir is descended from this special race and possesses, therefore, some of their unique characteristics.

Many lesser Maiar are said to have inhabited animal forms in Valinor. It is perfectly plausible to suppose animals could be given a "spirit" by those powerful enough to do so. The Thrush is said to be descended from a family of talking thrushes, they must have been endowed with their talking trait through unrecorded History.

My other guess is that Radaghast had some influence on using some animals for good (remembering that the Istari had been in ME for over 2 thousand years). He may have imparted (acting as an agent) some of his maiaric spirit into certain animals for some (mostly) unknown purpose.

There are tantalizing enigmas that Tolkien leaves for us to make our own mind up about.