It is just how the language has developed - nobody sat down and designed it! Different forms might come from older words no longer used, perhaps. In summary, though:
- Informal singular - Cer! or Dos!
- Formal or plural - Ewch! or Cerwch!
There will be some other local variations, too, I expect.
- Cer/Dos o 'ma! - Get/Go away (from here)!
- Cer/Dos o 'na! - Get/Go away from there!
- Cer ma's/allan!, Dos allan! - Get out!
- Cer/Dos bant/i ffwrdd! - Go away!
And various other ruder ones involving scratching, or playing with your Granny - the Clwb Malu Cachu web-site had a collection of them at one time, which might still be there perhaps.