"[Plural] are one of them"

"Most of animals are not dangerous. Bears are not one of them."

The part you are asking about is correct (second sentence).

The first sentence, however, is not. There would be possibilities:

"Most of the animals are not dangerous."

or

"Most animals are not dangerous."

Contrary to French, English uses plural for «il y a», for example:

«Il y a trois ours»

becomes:

"There are three bears"

in English.

While talking, though, it's often that someone will start with "There is ..." and not correct themselves (no real need). This happens when what comes after is not yet fully thought of.

Next, the "one" in "one of them" refer to a group or category or species (notice that the word "species" is always plural).

They could also have written:

"A Bear is not one of them"

But that would be wrong in that we want to talk about all the bears, not just one specific bear. We could also rewrite that sentence like so to make it clearer grammatically:

"Bears are not one of these animals"

I would think we'd say it the same way in French:

«Les ours ne sont pas un de ces animaux.»

Post Scriptum: Bears can kill you, but most are not dangerous when well fed and unprovoked. The fact that we stand up makes bear think you want to attack them (because that's what they do when they fight between each other). Older bears are more likely to have trouble feeding themselves in nature and this is one reason for them attacking humans: they're hungry. Females with cubs will also attack if they think you are after their cubs.