And here I am... More than 2 years already have passed and yet I am still waiting for Echidna to become summonable. Oh and Typhon as well for the sake of their "Monster cuple energy!"

P.S. In Fate/Strang Fake it was stated that Scylla (and/or?) Charybdis can be summoned as servants and heroic spirit container can fit them, same goes for the Fenrir as he was mentioned as Rider in Extra. We need more Kaiju/Monster characters here!