Agnes

Origin: Harry Potter
Hair:
Eyes:
Species: Human
Birthday:
Residence: Jacques Street 11 (7-103)
Occupation:
Affiliation: Shifty Business
Talent: Magic, Shapeshifting (dog)
Medical notes: Was half stuck in dog shape prior to import
Hobby:
Languages:
Import Date: 5
Pets:

Agnes was a witch and patient in the Janus Thickey Ward at St Mungo's Hospital for Magical Maladies and Injuries, admitted sometime before Christmas of 1995.[1]
She had fur all over her head and barked instead of speaking. It is possible that she remained at St Mungo's for the rest of her life. Her exact ailment was unknown, but it was noted to be a similar effect to consuming Polyjuice Potion prepared with animal DNA.[1]
Penny Haywood spoke to Agnes in the Janus Thickey Ward in the 1990–1991 school year while on work experience at the hospital in her seventh year.[2]
In late December 1995, her Healer Miriam Strout informed that her son would visit her Christmas night.[1]

Behind the scenes
Given the description on Pottermore presents what happens if an Animagus transformation fails, it seems likely that this is what happened to Agnes, with her attempted Animagus form most likely being a dog.