Katherine Nadal, 28, said the dachshund did it when the boy was 5 weeks old on March 13, 2007.
Nadal tested positive for cocaine, methadone and a tranquilizer the day after the 2007 attack, a Child Protective Services spokeswoman said.
Investigators said they ruled out dog bites and accused the former Anahuac High School cheerleader of using a sharp instrument to remove his penis, The Houston Chronicle reported.
The boy's aunt, Patches DeShazo, who is caring for the child, said the toddler is recovering and still waiting for reconstructive surgery.
Nadal is in jail in lieu of $75,000 bail.