Many nouns in Welsh have a the plural form that is shorter than the singular form (i.e. the singular form looks like the plural form + affix).
For example:
Singular
- coeden 'tree'
- seren 'star'
- malwoden 'snail'
Plural
- coed 'trees'
- ser 'stars'
- malwod 'snails'
This way of forming the plural is common in Welsh nouns. It may be that this way of forming the plural is fairly common in other languages. Even if this form of pluralisation isn't all that rare, I'd still like to know what this kind of pluralisation is called?