As people had been suspecting from recently released preview footage of Episode Duscae, the summons available in the Final Fantasy XV demo is Ramuh. Watch his summons animation, in which he uses his signature Judgment Bolt, above.
Ramuh is a wizened mage with a glorious beard and staff, who uses lightning attacks. He appears in Final Fantasy XV as a gargantuan humanoid. (Footage from years ago shows that Leviathan in FFXV is also enormous. Are all the eidolons this big?) He will only appear to Noctis if the Prince is low on health, and will heal Noctis in addition to executing his attack.
The most recent Final Fantasy game in which he appeared was Final Fantasy Legends: The Space-Time Crystal. Of the main Final Fantasy titles, Ramuh has also appeared in Final Fantasy III, IV, V, VI, VII, IX, XI, XIII, and XIV, and in even more side titles. When he’s absent from an FF game, Quetzacoatl typically takes his place.
You still have about two months to buy Final Fantasy Type-0 HD and install/download the Episode Duscae demo, after which point it will no longer be available.