User blog comment:Palantian/Fate/Oblivion Twilight/@comment-2152539-20150206084548/@comment-4616657-20150206200954

Well, like I said, Puck is a Fairy. Though, Loki is sometimes descibed as a Giant, God, or both. If taken from the Giant category, he can be treated like Chiron and Medusa as a Human-esque Mythical Creature possible to be summoned. But Gods themselves can't be summoned. Karna and Hercules did indeed fuse with Gods or become Gods after death, but they're still summoned in the Prime of their lives. So, while I don't find either particularly usable as Servants, Loki does seem the more logical choice of the two if one has to be used.