Melvonia Plonk was the "Madame Plonk" at the Great Tree before Brunwella. She was well known in the Northern and Southern Kingdoms. According to Brunwella, she was very young when she died. It is possible she was murdered by Rodmilla.
Lost Tales of Ga'Hoole: The Snowy Sisters
The death of Melvonia was spread. Rodmilla, Brunwella and Thora's stepmother, was thrilled by the news so she could move on with her plans as a slipgizzle.