It's one of those objects where the basic form refers to several objects -- mostly objects such as fruit (and children!) that are usually encountered in groups.
The singular is often formed by adding -en if they're feminine (llygoden, mefusen) or -yn if they're masculine (plentyn).
Oh dear, didn't know until now that Welsh also has masculine and feminine nouns!
It does :)
It doesn't have separate articles for them as French does (le, la); but the difference is still significant in grammar because, for example, adjectives mutate softly after a feminine noun but not after a masculine noun: mefusen fach "a small strawberry" but plentyn bach "a small child".
So it's best to learn the gender along with the noun if you can.