The poem The Creature/John Clare recites at the end of the episode is called "Ode: Intimations of Immortality from Recollections of Early Childhood" by the English poet William Wordsworth. In his poem, Wordsworth laments his lost connection to nature as he grew older and humanity's failure to recognize the value of the natural world.
This episode is effectively the series finale as creator John Logan always envisioned Penny to be a 3 season show.
The sign on the old slaughterhouse is Hinderaker. One of the credited staff writers is Andrew Hinderaker.
"The blessed dark" are the final words of the Ursula K. Le Guin short story "The Wife's Story" in which the wife's husband is able to shift from a wolf into a man.