This is really weird IMO; the plural of cactus is cacti, so why is not the plural of chorus, chori?
marked as duplicate by Robusto, Uticensis, kiamlaluno, MrHen, F'x Apr 21 '11 at 13:35
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The plural of "ox" is "oxen", so the plural of "box" is ...
The plural of "man" is "men", so the plural of "can" is ...
These are no odder than your question.
English has a default rule for forming plurals ("add 's'), but lots of other rules (eg "-y" -> "-ies") and individual exceptions. Analogy is a significant force in language change, but it is not predictable when it will have effect and when it won't.
As mplungjan says, "chorus" and "cactus" are from different languages, but that doesn't really answer the question: in Greek "khoros" would have plural "khoroi", but we have hardly any examples of the Greek plural "-oi" in English. On the other hand the plural of the Greek "oktopus" is "oktopodes", and some people use "octopodes" in English.
And even in words coming from Latin, simple analogy doesn't always work. The plural of "corpus" is "corpora" in both Latin and English. And the plural of "prospectus" in Latin is "prospectūs" - the usual plural in English is "prospectuses".
As pointed out in this previous general question, there's no real rule.
Chorus entered the English vocabulary during the christianization campaign started in 597 with St Augustine that would make of England one of the most developed Christian country in the Middle Ages. Together with cloister, mass, alm, organ, sound etc.
Latin took it indeed from the Greeks (χορός - choros) but the Greek plural would actually make it choroi (χοροι).
The English plural ignores the Latin origin of the word.
As for cactus, it also comes from Greek (κάκτος) through Latin (cactus). However, this time the Latin plural is respected. May be cactuses was really too prickly !!! I first though that it would be Linnaeus's fault but you don't say gerania but geraniums. So bad bet !