As @JohnLawler said, not all Latin words keep their Latin plurals. Consider:
- virusvirus/viruses (not virii, see link)
- fetus/fetuses
- victor/victors
- vector/vectors
- minus/minuses
- onus/onuses
- sinus/sinuses
- status/statuses (yes, statusesyes, statuses)
- stadium/stadiums (Greek word actually but came to English through Latin and (almost) nobody says stadia)
And the list goes on. I have no Latin so I don't know how many of these don't keep their plural because of the rule mentioned in @PeterShor's answer but you can see that many Latin words have normal English plurals. In other words, trust the dictionary, it tends to know more than we do.