http://i.imgur.com/WUL6UKh.png
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It is plural. Like Cactus > Cacti
Yea I had a few of these with silghtly different naming at the end, little bit confusing but luckily they're distinctive from the symbol above their heads :P
Many Garlean terms are Latin. Inflectional morphology in Latin is complex, but all the examples I've seen in-game are correct. So it would be "I killed a hoplomachus. Now I've killed ten hoplomachi."