In the realm of Supernatural Sam fanfiction, it is extremely unlikely to find any exploration of bestiality, even symbolically. The Supernatural fandom values the integrity of the show's themes and characters. Sam is a key figure in a world filled with angels, demons, and other human hunters. Fanfiction writers are more interested in exploring his role in the battle against evil, his family ties, and his personal growth. Bestiality, in any form, does not fit into this framework of storytelling.
I highly doubt it. Supernatural fanfiction, especially those centered around Sam, tend to revolve around his character development, his relationships with other hunters or supernatural beings in a more traditional sense. Symbolic bestiality would be a very strange and out - of - place concept in this context. It would deviate so much from the norms of the Supernatural world that it's hard to imagine any fanfiction writer going down that route.