The phrase "nervous mouse" typically refers to someone who is anxious, fearful, or timid. Antonyms for this phrase could include "confident lion," "bold eagle," or "brave tiger," representing individuals who are self-assured, fearless, and ready to face any challenge. Another possible antonym could be "relaxed cat," representing someone who is calm, patient, and at ease in any situation. Choosing a different animal to represent the antonym can help to create a clear contrast and convey a contrasting personality trait. Regardless of the chosen antonym, the important thing is to communicate a sense of positivity and strength in opposition to nervousness or fear.