Could you please explain what the semantic difference is between two sentences? Are they both idiomatic?
There is a fox running along the road.
A fox is running along the road.
Or we have to use "there is" when when we talk about something for the first time. And it works all the time! For example, "There is a man running along the road." But, "He is running along the road.", because we know that person in some way, so we don't need to use "there is". But this fox we can't know (if we aren't workers of the zoo and the fox has ran away from the zoo) and "there is" is always necessary when we see a new animal or person. Right?