Notes:

Notice that a lot of these titans failed to appear in the movie – including titans that were a lot closer to Boston than those who did show up. Fortunately, I have some answers:

Leviathan – got into a bar fight with a Glaswegian (RIP)
Sargon – abducted by aliens in exchange for Richard Dreyfuss
Tiamat – recruited by the GOP as their candidate for Georgia Senator
Quetzalcoatl – went to New York to lay an egg
Mokele-Mbembe – got sucked into a "we wuz kangz" troll thread on 4chan
Baphomet – went off to re-start the Knights Templar
Sekhmet – distracted by a giant laser pointer
Abbadon – drank too much vodka (his liver RIP)
Yamata no Orochi – died in battle with scrappy teenage ninjas
Typhon – trapped under a volcano by Zeus
Bunyip – accidentally sank Twilight Sparkle's boat, and then stuck around to apologize