The spelling
syllabus came from a fifteenth-century edition of Cicero’s
Letters to Atticus where it is a misspelling of a form of the word
sittybus, which refers to a strip of parchment. However, now
syllabus is thought of as a Neo-Latin word with a plural form
syllabi, so it is a second-declension masculine noun.
If you chose to use “syllabi” in English, and someone says it is wrong, you could say that the word is now, regardless of its origins, a Neo-Latin word, and such a plural may be formed.
From
Plûrâlitâs Latîna