"The squirrel has a long tail."
Translation:Ekorren har en lång svans.
I am a native Swede and I would absolutely omitt 'en', and say 'Ekorren har lång svans', I would also say 'Ekorren har svans', as opposed to animals without tails. It is a general statement. --- On the other hand, if I find a squirrel with a unusually short tail I would probably say 'This squirrel has a short tail'. It is specific statement, something out of the ordinary.
Ah, I thought it was about the English version. In Swedish you can drop the article for things that you are supposed to have, like body parts, if there's an adjective before it. If anything, it sounds better without the article.
Hon har stor näsa or Hon har en stor näsa = She has a big nose
Han har stor mun or Han har en stor mun = He has a big mouth
Jag har lång hals (or possibly Jag har en lång hals, this does not sound very good to me though) = I have a long neck
Hunden har lång svans or Hunden har en lång svans = The dog has a long tail
All these body parts that people or animals are expected to have can be said without en/ett in Swedish. ’Everybody’ has a nose, a mouth, a neck etc, so the focus is on what the nose etc. is like.
PS: If you're talking about things that it is not taken for granted that someone should have, you can't drop the article.
Han har ett stort ärr på armen = He has a big scar on his arm
you can't drop ett because the presence of the scar is information in itself.