"I know what we're gonna do today. (laughs maniacally)" — Dr. Phineastein[source]
Dr. Phineastein is an ancient Victorian scientist who created a monster platypus with the help of his hunchbacked henchman, Ferbgor. Dr. Phineastein lived in Drusselstein with Ferbgor.
Nothing is known about how Phineastein grew up, but he appears to have created several monsters in his time. One monster was a two headed cow, but it either disappeared or ran away. He lived in a castle governed by a disapproving governess named Constance, who was possibly related to him and who, like Candace, constantly tried to "bust" Phineastein and his henchman Ferbgor by telling on them to the Angry Mob ("The Monster of Phineas-n-Ferbenstein").
Creating a Monster
At some time, Phineastein was gathering spare parts from platypi, when he saw a flier for a "Monster Ball". Inspired to enter it, he and Ferbgor created a new monster, this time, a platypus one.
In the monster ball, he finds the monster, who has just eaten Dr. Jekyll Doofenshmirtz. Just then, Constance arrived, with the angry mob following. Constance saw the potion brewer that Dr. Jekyll Doofenshmirtz built, and took the drink from inside it, and drank it. She turns into a monster, as a result. Just then, the winner of the Best Monster Contest is announced, and Constance is the winner ("The Monster of Phineas-n-Ferbenstein").
Future and Legacy
While nothing is known about the rest of his life, it is made clear that he is dead as of the present time. Phineastein's legacy lives on through the memories of Reginald Fletcher and his grandchildren of whom he told his story to.
Dr. Phineastein looks exactly like Phineas Flynn, but he is a scientist, unlike his original counterpart. He has never been said to be related to anyone in the Fletcher family. But he is related to the Flynn family and seems to be related to Constance.