There are three likely book series with Pegasus in the title/series;
Kate O'Hearn's books:
Pegasus and the Flame (Pegasus, #1) Pegasus and the Fight for Olympus (Pegasus, #2) Pegasus and the New Olympians (Pegasus, #3) Pegasus and the Origins of Olympus (Pegasus, #4)
No fifth title mentioned.
Anne McCaffrey's Talents series:
To Ride Pegasus (1)
Pegasus in Flight (2)
Pegasus in Space (3)
It is followed by the The Tower and the Hive series:
The Rowan (1)
Damia (2)
Damia's Children (3)
Lyon's Pride (4)
The Tower and the Hive (5)
Get Off the Unicorn also has short stories involving this series.
Robin McKinley's Pegasus books,
Pegasus (1)
Ebon (2) - to be published 2015
The Golden Country (3)-to be published 2015
As of 2010 there are only four books, we will have to wait and see if there will be a fifth book.
The fifth book is called "A Dangerous Path".
Pegasus was/is a mythological creature and does not have a scientific name.
The name of the fifth book was Waterwings but they changed the name to MAX
everlasting is the name of the last book
Pegasus is a Greek figure.
The fifth book in The Lightning Thief series is "The Last Olympian."
the name of the 5th 39 clues book is the black circle
equus-avis is the latin name for pegasus
The second book in the Pegasus series by Kate O'Hearn is titled "Pegasus: The Flame of Olympus." In this installment, the story continues to follow the adventures of the main characters as they navigate challenges in a fantastical world involving gods, mythical creatures, and their quest to save Olympus.
The name of the winged horse is Pegasus. Pegasus is also only Hercules' in the Disney movie, in Greek myth Pegasus belonged to the hero Bellerophon.
The Last Olympian