I'm not familiar with irregular Latin pluralization, so this may be a simple question with a simple answer. Other Latin words ending in "us" don't pluralize to "era"
English has borrowed many words from Latin and often the plural in the original gets carried along, instead of just using the English plural.
- usually words ending in -us (second declension) have a plural in -i, and that is the most common 'true-to-latin' plural in English is: eg alumnus -> alumni (though pronounced English style: uh-LUM-nigh, different from the more Latin ah-LUM-knee).
- but the plural, in Latin, for some Latin neuter nouns ending in -us is different because they fall under the third declension, with its own set of endings. -a is the nominative plural ending for third declension neuters), and under which the final -s undergoes the Latin plural sound change called rhoticization changing s to r: opus -> opera, which means work -> works, an opera is originally a collection of pieces. This is the answer to why the -Latin- plural has 'r' instead of 's', because it was in the transition in Proto-Italic between Old and Classical Latin when the rhotacization occurred.
Opus/opera, genus/genera, corpus/corpora are the relatively more common examples, the others are much rarer (viscus/viscera).