You can add your own pluralizations to Rails by adding inflections. Rails should have a file called inflections.rb under /config/initializers. You can add it there if it isn't. My file as an example (comments come from Rails):
# Add new inflection rules using the following format
# (all these examples are active by default):
# Inflector.inflections do |inflect|
# inflect.plural /^(ox)$/i, '\1en'
# inflect.singular /^(ox)en/i, '\1'
# inflect.irregular 'person', 'people'
# inflect.uncountable %w( fish sheep )
# end
ActiveSupport::Inflector.inflections do |inflect|
inflect.plural(/rion$/ ,'ria') # criterion => criteria
inflect.singular(/ria$/, 'rion') # criteria => criterion
end