Tonic is a unicorn NPC located underneath the Waterfall Tree in northwestern Ponydale. She is the town's alchemist. She is one of the three ponies players must speak to during the Supplying the Hospital quest, as she gives the Medical Potion Crate quest item.
She also has her own quest, Tonic's Tonic, which can be completed an unlimited amount of times.
"Name's Tonic. I'm the alchemist around here. If you ever need something brewed, just hit me up."