Good news: both of your sentences are almost right! Here are clearer, correct versions a 6-year-old can read easily:
- A narwhal's horn is actually a very big tooth called a tusk.
- What looks like a narwhal's horn is actually a very big tooth called a tusk.
Why these are better: adding the word "actually" makes the meaning clearer. The first short sentence is the simplest to say and understand.