The Nemean Lion appears during the song Zero to Hero. He is one of the monsters sent by Hades to kill Hercules, but he is easily defeated.
Hercules: The Animated Series
Hercules and the Hero of Athens
After getting bored of Nemea, the the Nemean Lion goes to Athens to spread terror. He enters in the Great Hall of Pimentos and attacks the employees, but Hercules hears their screams and comes to their rescue. However, he accidently destroys the building when he arrives and the Nemean Lion is knocked out and captured. Icarus takes the blame for the damage to protect Hercules, thus making everyone believes that he is the one who defeated the Nemean Lion, including the Nemean Lion himself.
A few days later, the Nemean Lion escapes and returns to Athens in order to take revenge on Icarus. Hercules and Icarus comes to face him and Hercules manages to knock him out again, but it's Icarus who takes the responsibility for defeating him once again.
The Nemean Lion later escapes one more time and confronts Icarus. However, after a short fight he realizes that it was Hercules who defeated him and tried to eat Icarus. Hercules then arrives and fights the Nemean Lion, who eventually beats him. Before he could kill Hercules, Icarus comes to help his friend and they manages to defeat the Nemean Lion.