SamuraiPegasusPlurals

It depends completely in whose faux rules you want to follow and why.

For starters, there actually isn't a single mythological species known as a pegasus, they are called pterippus and were fairly famous as a sign of being blessed by the gods, after all it is a flying horse.

Now, because people are less than original, pterippus is weird sounding, and Hercules is more than famous enough, people immediately related this kind of creature to Pegasos, his horse.

Now, here is where it get's tricky: because Pegasos is a proper name and was originally in greek, the transfer to english occurred as part of a messy amalgamation of both latin and greek terms into a single and undivided entity. This as a result came to make the rules of capitalization which rule each of their respective language get... confused.

Seeing how pegasus was created, people began to use latin rather than greek capitalization for the word, because most if not all greek words which ended in os got turn into their latin equivalent that ended in us and thus we ended with a greek word which is pluralize in latin (again, the original word is a greek name, Pegasos, which got turn into a noun, pegasos, which then got turn into latin, pegasus).

So now, for the pluralization of latin words, the plural of any word ending in -us it is -i (focus, foci) and thus if you are to remember all that happened above, you will end up with pegasus as a singular and pegasi (faux latin at it's best) as it's plural.

But wait! There is also the fact that the word in question is still greek in origin and the rules of latin pluralization have no reason to fit in with it. Seeing how the rules of pluralization of the noun version of the original name is in greek, despite all the changes, it is actually arguably better to say the plural is pegasoi/pegasai, as that is the plural for the original word (still faux greek though).

But wait! There are other things at play as well, namely the fact that the word might or might not be a different declesion that the one we expect, due to the fact the word never existed in both languages we try to bring it from, there is a good chance other rules of pluralization apply to it, making pegasudes be equally as valid depending on how much deeper you want to go.

But wait! Considering the fact pegasus is an english construct rather than any real greek or latin word, this would make it be a completely original term (if based upon greek and latins words) and make it so that its pluralization be exactly like any other word. Thus, pegasuses is then again just as valid as any of the above.

And thus, you end up with a gigantic mess of possible translation, pluralizations and spellings, all because people couldn't wrap their heads around pterippus (but could around a hippocampus.)