they named their pets after characters from tv shows that they both watched as teens and have a deep connection with. Willow is their cat, from Buffy the Vampire slayer, the groundbreaking show that had strong female leads and was the first network show to include openly gay characters in relationships that weren't hidden or gratuitous. They even included songs from the musical episode in their wedding!
Piper comes from Charmed, yet another show with strong badass female lead characters. They toyed with naming her Pterodactyl, and then using the shortform of Ptero, pronounced Tara, since she was willow's love interest but realized they would constantly be explaining