No, since "A dog" in Welsh is "Ci", therefore "A dog" is an incorrect translations of "Un ci".
So would you ever use Un Ci and not just Ci ? I feel this is more a critique of English, I would rarely ever say one dog therefore a valid translation in English would be a dog, as in "look there is a dog" but "look there is one dog" sounds wrong.
Well you you wouldn't say "Edrycha mae na un ci", though you would say "I have one dog, and two cats" (Mae gen i un ci a ddwy gath).
That is fine in Welsh then but I would say "I have a dog and two cats" in english I just would never use "one" dog unless I was counting them, it sounds wrong and thus I still think it is an acceptable translation?