Latin was my first foreign language, and I learned it at school for six years (now all the German users know which type of school I attended
There are two ways of creating the plural of a word ending in "-us".
1.: o-conjugation - dominus -> domini
2.: u-conjugation - ictus -> ictus (with a long u-sound)
So, without knowing whether cactus and octopus are o- or u-conjugation, I can´t tell whether their respective plurals are cacti/octopi or cactus/octopus.
P.S.: The ethymologically correct plural of "forum" is "fora".