I believe the answer to the riddle is "Moaning Myrtle," a character
from the "Harry Potter" books. Moaning Myrtle is an emotionally
volatile ghost ("apparition dotty") who resides in a toilet
("sequestered to a potty") in the girls' bathroom of Hogwarts. Myrtle
was killed by the gaze of a scaled, dragonlike beast, the Basilisk
("scaly stare so haughty").
"Moaning Myrtle is a ghost who haunts a toilet in a girls' bathroom on
the second floor (see) of Hogwarts castle. Myrtle was a student at
Hogwarts in 1943 [Y-37] when she was killed by the Basilisk from the
Chamber of Secrets. She had been hiding in that bathroom because she
was being teased by Olive Hornby. Then she heard someone come in and
heard that person speaking. She realized it was a boy, so she opened
her stall to tell him to get out of the girls' bathroom, only to find
herself face to face with the Basilisk. Myrtle decided to haunt Olive
Hornby to make her pay for teasing her so much. She followed Olive
around until the Ministry had to step in and stop her. So Myrtle
returned to her toilet and moaned and cried and splashed water out
whenever she felt particularly miserable."
Harry Potter Lexicon: Moaning Myrtle
"Myrtle (surname unknown) is the ghost of a Muggle-born witch who died
while a student at Hogwarts, fifty years prior to the events in the
books. An unhappy girl, she would sometimes retreat to the second
floor girls' lavatory to weep. Coincidentally, the entrance to the
Chamber of Secrets was hidden in this bathroom. On one occasion as
Myrtle was hiding there to elude Olive Hornby, a classmate of hers who
perpetually tormented her with teasing, the Chamber was opened, Tom
Riddle's basilisk emerged, and it slew Myrtle.
After death, Myrtle haunted Olive everywhere she went, harassing her
in revenge for the teasing Myrtle had endured from her in life...
Olive complained to the Ministry of Magic, who ordered Myrtle to
return to Hogwarts. Ever since then, Myrtle has haunted the same
lavatory where she died."
Wikipedia: Moaning Myrtle
Google Web Search: "harry potter" "moaning myrtle"
I hope this is exactly what you're looking for. If it is not, please
request clarification; I'll gladly offer further assistance before you
rate my answer.