Quick Guide to Latin Pluralizations in English that You Never Asked for:

If it ends in a -us, it becomes -i:
radius → radii
alumnus → alumni
nucleus → nuclei

If it ends in an -a, it becomes -ae:
alumna → alumnae
formula → formulae
vagina → vaginae

If it ends in a -um, it becomes -a:
stratum → strata
medium → media
datum → data

If it ends in an -is, it becomes -es:
axis → axes
crisis → crises
genesis → geneses

Exceptions (Well, not in Latin. It makes sense in Latin):
opus → opera
appendix → appendices
stigma → stigmata
index → indices
corpus → corpora

Questions I am actually asked:
1. Yes, penis ends in an -is, so the plural is actually "penes," but no one will know what you're talking about if you say it aloud.
2. Octopus is not actually Latin but Greek, so the real but completely unused plural is "octopodes."
3. It's cacti.