I agree with Dylan, this is not a normal conversation word. Also you wouldn’t say “in a consternation”. The word requires some different constructions. Here are some examples: I was feeling some consternation when I saw my report card. To my consternation, my metro station was closed with no notice. The latter example is the closest to something I would say in conversation, but I’m having a hard time thinking of a scenario where it would sound natural and not pompous. Writing can commonly be more formal though. Maybe i would use it in conversation if I were talking with someone who used an elevated level of speaking and word choice and it seemed they would be more comfortable if I did as well. But that’s really rare.

I would agree with Dylan. I'm not sure I have ever used the word.

This word is almost never used outside of literature, you would be better off saying " I was frightened when I saw this fierce dog"