In Greek mythology, there was only one Pegasus: that’s his personal name, not a species name. But if you did have more than one, and you wanted to get the plural form really, truly correct, it would never be pegasi, it would be pegasoi.
Pegasus comes from Greek, not from Latin, so the Latin ending is not authentic.
In fact, sticklers really ought to be writing the singular form as pegasos or even pégasos. If you’re going to be pedantic, do it right.














