You can cast a creature spell for its blitz cost only if you could cast that creature spell
You can, however, pay additional costs

The Final Days Gaius van Baelsar Hecteyes Jecht, Reluctant Guardian Kain, Traitorous Dragoon Malboro Namazu Trader Ninjas Blades Overkill Phantom Train Poison the Waters Qutrub Forayer Reno and Rude Resentful Revelation Sephiroth, Fabled SOLDIER Sephiroths Intervention Shambling Cieth Shinra Reinforcements Sidequest: Hunt the Mark Summon: Anima Summon: Primal Odin Tonberry Undercity Dire Rat Vaynes Treachery Vincent Valentine // Galian Beast Vincents Limit Break Zenos yae Galvus Zodiark, Umbral God Barret Wallace Blazing Bomb Call the Mountain Chocobo Choco-Comet Clive, Ifrits Dominant Coral Sword The Fire Crystal Fire Magic Firion, Wild Rose Warrior Freya Crescent Gilgamesh, Master-at-Arms Haste Magic Hill Gigas Item Shopkeep Laughing Mad Light Of Judgement Mysidian Elder Nibelheim Aflame Opera Love Song Prompto Argentum Queen Brahne Random Encounter Raubahn, Bull of Ala Mhigo Red Mages Rapier Sabotender Samurais Katana Sandworm Seifer Almasy Self Destruct Sidequest: Play Blitzball Sorceresss Scheme Summon: Brynhildr Summon: Esper Ramuh Summon: G.F

This will not only let you get multiple uses out of Giant Cobra, it will also keep your hand well-stocked, giving you lots of cards to discard to the Mysterious Advantage ability for tasty, tasty, game-winning lore
Unlike character cards, however, they dont need to be exerted in order to be challenged as they are always exerted from the moment they are played and they dont ready (hence the horizontal way they are printed)
If you play other card games like Magic The Gathering and Pokemon you can also check out our rarity guides