Perry the Platypus

Perry the Platypus is the Flynn-Fletcher family pet. He's secretly a secret agent codenamed Agent P.

Ex-Human
Some fans believe that Perry the Platypus used to be a human.

Supporting Evidence

 * Longer lifespan ("Phineas and Ferb's Quantum Boogaloo")
 * Sentient
 * Ability to dream, contrary to the popular belief that platypi are unable to dream ("Phineas and Ferb Get Busted!")
 * Using breathing aids underwater ("The Magnificent Few")
 * Dislikes eating bugs ("Brain Drain") and enjoys human food
 * Possibly human-sized equipment in the background of Perry's lair

Opposing Evidence
None at the moment.

Phineas and Candace's biological father
Some fans believe that Perry the Platypus is Phineas and Candace's biological father. This was first brought up by Bobtherandomguy in one of his posts, which was then developed by various theorists including Lala Stellune and NotAGothChick101. In Bobtherandomguy's theory, Perry got injured and had his soul transplanted into a platypus. Lala Stellune said that Perry turned into a platypus in purpose after a major conflict with Linda Flynn.

Supporting Evidence

 * Perry used to be a human being
 * Perry's affection to Phineas and Candace:
 * Knew a lot about Candace ("Phineas and Ferb Get Busted!")
 * Possibly shared a dream with Candace ("Wizard of Odd")
 * Cheat using robot mechanics to help Candace win ("I Was a Middle-Aged Robot")
 * Protective towards Phineas, Ferb, and Candace ("Journey to the Center of Candace", "Phineas and Ferb The Movie: Across the 2nd Dimension")
 * Sleep on their beds, even Candace's.
 * Similarities between Perry and Phineas:
 * Perry has mechanical ability ("Phineas and Ferb Hawaiian Vacation", "I Was a Middle Aged Robot", "Phineas and Ferb Interrupted")
 * Perry has musical ability ("The Lizard Whisperer", "Phineas and Ferb The Movie: Across the 2nd Dimension", "Skiddley Whiffers", "Brain Drain")
 * Similarities between Perry and Candace:
 * Perry has musical ability
 * Candace can do martial arts and has agility ("Fireside Girl Jamboree", "Phineas and Ferb The Movie: Across the 2nd Dimension", "Thaddeus and Thor")

Opposing Evidence
None at the moment.

Fanfiction

 * Starring Perseus Flynn:
 * Short Stories of the Vanished Man
 * What a Man
 * Who's My Daddy?
 * Starring Phelan Flynn:
 * Phineas and Ferb Space Wars: Episode 4
 * The Fate of Phelan Flynn
 * The Bitterness of Candace Flynn
 * Mingled Sense and Nonsense

Articles

 * Who is Father Flynn? by Bobtherandomguy
 * Lala's Long but Orderly Put Theory about Phineas and Candace's Biological Father by Lala Stellune

Perry's Gender
Despite that many characters mentioned that Perry's male, some fans think that Perry is a girl.

Evidence that Perry is a Boy

 * Many characters mention that Perry is a "he".
 * Five-o-clock shadow ("Oil on Candace")
 * Perry wore clothing suited for males, including boxers ("Perry Lays an Egg"), a suit ("Undercover Carl"), and male costumes ("She's the Mayor").
 * Perry went into the boy's bathroom ("My Fair Goalie")
 * Perry used his poisonous spurs against the Liam, the Platypus Hunter. (Primal Perry)

Evidence that Perry is a Girl

 * When Candace switched bodies with Perry, she made Perry's body sweat milk ("Does This Duckbill Make Me Look Fat?"), and his sweat is also white in various episodes.
 * Phineas, Ferb, and Candace thinks that it's possible for Perry to lay eggs