Matilda is a princess representing an officially unknown species.
Though Matilda appears as a background character at various points throughout the series, her only spoken appearance is at the start of the episode Lost, where she greets Polvina and Ester as they are going to their first day of school.
Speculation[]
Judging by the appearance of the fish accompanying Matilda, which have orange backs, white bellies and yellowish-gold fins with a black stripe, it can be speculated that the fish are freckled hawkfish, meaning that Matilda is likely to be the Hawkfish Princess.