The missing / Kate O'Hearn.

Enhanced descriptions from Syndetics:

Follow Astraea, Zephyr, and their friends as they travel to a dangerous jungle world in this second book in a brand-new series from bestselling Pegasus author Kate O'Hearn, who masterfully blends mystery and mythology together.

With the weakened Titan prisoners hidden on Earth, Astraea, Zephyr, and their team head to Nesso's jungle world to seek help in the struggle against the Mimics.

But nothing goes to plan. The moment they arrive, they encounter a world filled with dangers they never imagined possible--from massive, hungry dinosaurs to a blisteringly hot sun. Soon Zephyr and Astraea are seriously wounded. Without ambrosia or nectar to heal them, the group soon realizes they are in serious trouble. But where can they go for help?

With Zephyr's life slipping away, Jake and Tryn use their new flying skateboards to try to make it to Xanadu where they hope to find ambrosia for Zephyr and enlist help to join the fight against the Mimics.

Meanwhile, Astraea and the three centaurs must use all their cunning to keep Zephyr safe and alive. But danger comes in many forms, and it seems as though no one may be left to help them--until Jake and Tryn stumble across a lone Olympian everyone believed to be dead...