Some words follow patterns for pluralization and some do not.

crisis becomes crises and hypothesis becomes hypotheses

appendix becomes appendices and matrix becomes matrices

child becomes children and ox becomes oxen

bacterium becomes bacteria and curriculum becomes curricula

antenna becomes antennae or antennas and formula becomes formulae or formulas

louse becomes lice and mouse becomes mice

knife becomes knives and self becomes selves

echo becomes echoes and tomato becomes tomatoes

deer becomes deer and offspring becomes offspring

foot becomes feet and goose becomes geese