Jerry’s cousin in the cartoon “Jerry’s Cousin” is Muscles.
Muscles is introduced as a remarkably strong mouse who comes to Jerry’s aid when Tom threatens him. Unlike Jerry’s usual strategy of relying on clever tricks and evasive maneuvers, Muscles uses physical strength and an aggressive attitude toward cats, making him an especially intimidating opponent for Tom.
The short “Jerry’s Cousin” was released in 1951 during the classic Hanna-Barbera/MGM period. Muscles is presented as Jerry’s relative, but he is not the same character as Nibbles, Tuffy, or other young mice who appear elsewhere in the series.
Muscles had a later appearance in “Tom and Jerry: The Magic Ring,” where the character was renamed Freddie. Because that version has a different name and slightly different context, some listings treat Freddie as a separate or reworked character. The original cartoon’s answer, however, is clearly Muscles.