What is the collective noun for pegasi? I know for a fact that the plural noun is pegasi but what is the collective noun?
There is no standard word.
Pegasus, from ancient Greek mythology, was a singular god-like creature whose father was Poseidon, god of the seas.
In more modern times, where writers introduced the idea of the pegasus being a species of mythological creature, they still tended to show up here or there and not in an entire group.
And so there isn't a standard word to use for this.
Which now brings us to the question of what you could call a group of pegasi.
There are a few ways you could go with this:
a focus could be made on the horse-like features of the pegasi and so use a word such as "herd".
a focus could be made on the bird-like features of the pegasi and so use a word such as "flock".
that said, a group of unicorns, which are arguably more horse-like than pegasi, are called a "blessing". And so a focus on a different aspect of pegasi could be used to guide a term. Something that comes to mind for me when thinking about Pegasus is his nobility, so perhaps "a nobility of pegasi".
and lastly, with no standard term, you can coin one. So long as you use it with conviction and make it clear that the word refers to a group of pegasi, you'll be fine.