Great sentence idea! Here are two correct ways to write it:
- Plural (more than one): Echidnas feel safe in their spiky superhero coats.
- Singular (one): An echidna feels safe in its spiky superhero coat.
Why? If you mean more than one echidna, use echidnas and their and coats. If you mean one echidna, use an echidna and its and coat. Also remember to start the sentence with a capital letter and end with a period.