The Roc originates from Persian mythology, and was featured in the classic tale: Sinbad the Sailor, and many media renditions or references to it. They are portrayed as birds of prey big enough to carry off whole full grown elephants. The one shown here is accurate to that size. Judging by retrospect to Zecora, Twilight Sparkle, and Rarity, it's about 50 feet in height with a wingspan of about 180 feet.
Friendship Report: Spike learned that even though he is growing up, it doesn't mean that Twilight is going to make him leave anytime soon.
Pee-Wee, the Phoenix who Spike adopted in "Dragon Quest," makes a return appearance and is seen all grown up.
Rarity mentions that if and when Spike goes to visit Zecora about his stone scales, she recommends that he doesn't use her shampoo. She was probably referring to what happened to her back in "It Isn't The Mane Thing About You."
Spike turns to stone, due to a rock solid cocoon. His superior, Twilight, previously met this fate in "Stare Master," as both friends survive these similar dangers. The only difference is whereas Twilight is rescued by Fluttershy, Spike breaks out of his cocoon with newly sprouted wings.