Late reply, but I definitely chuckled when I saw this word for the first time. To me it just sounds like he's sitting on his...you know what. Sounds uncomfortable. There was another sentence, something like "the squirrel has a big tail" which just had me thinking of a small squirrel with a huge package. XD
You say "svanns" not "svans" in swedish. It spells with one n but you say it as if there were two, meaning it is a short a. "svans" could actually mean it belongs to a swan, for ex: "Det var en svans ägg"= it was a swans egg. Do not confuse with it was a swan egg which would be "Det var ett svanägg"
Not a lot, but it is not that uncommon either. Some from the top of my head: ett får/sheep, lejon/lion, marsvin/guineapig, ostron/oyster, rådjur/deer.
Words specifically for a male or female animal are always en-words (i think) but generic words and kids can be ett. E.g. "en tacka/en bagge" which is f/m sheep (but ett lamm=lamb)
I think the reasoning when this was created was that the lion could be on its tail without actually sitting on it, so since the Swedish sentence specifies that it's sitting, it shouldn't be translated as just "is". After all, Swedish could have just said lejonet är på sin svans for the less specific option as well.
I'm not entirely opposed to the idea of accepting both, but I think it's probably slightly better to keep the meanings separated.