Fairy Tail Cross Over: Natsu x Lucy is Eugine and Rapunzel from Rapunzel, Gray x Juvia is Jack Frost and Elsa from Rise of the Guardians and Frozen, Jellal x Erza is from Epic (i don't remember their names XD) and Gajeel x Levy is Hiccup and Astrid from How to Train your Dragon.
Plue aka Nikora, "The Canis Minor" (ニコラ Nikora) is a Silver Key Celestial Spirit that has no obvious combat abilities and is stated to usually be kept as a pet for low-powered Mages, but is as powerful as any other character in the game Fairy Tail Gekitotsu! Kardia Daiseidou. Lucy Heartfilia seems to use hers, named Plue, as a pet and a comical character. Nikora keys are shown to have been sold for as little as 20,000 Jewel.