Here's another odd plural. The plural of genius is... genii. Yes, most people use geniuses but it's more accurate to use genii. (It's pronounced gee-nee-i). Unusual plurals like genii tend to get abandoned over time as people assign more common plural forms to these words. I can understand the desire to make plurals more standaradised. It also makes English easier to learn. But then again, having quirky words like genii makes the language interesting and helps to reveal the history of the word.
Genius has Latin roots and hence the unusual plural. We can tell a word probably has Latin roots when you study their plurals. Other Latin words with unusual plurals include
Stadium (plural: stadia)
Gymnasium (plural: gymnasia)
Formula (plural: formulae)
Antenna (plural: antennae)
The list goes on. You get the idea.

