This
painting
is
often
explained
as
an
allegory
of
the
war
in
Spain,
either
as
a
figure
representing
Spain,
or
of
the
crushing
power
of
war
itself.
Goya
used
a
combination
of
brush
and
palette
knife
to
complete
this
work.
Interestingly, in regards to arguments disputing the authorship of this painting to Goya, to consider also is the drawing and the mezzotint etching of what appears to be the same subject, and are grouped together by most past Goya writers. See information below to read more on the issues of attribtution, and also to see the drawing and mezzotint.
News Archive Items
UPDATE: FEB 2010: Dr Sarah Symmon addressed the issue of the bull "anatomy" in an article in defense of the Colosus being painted by Goya:
When the horses were important, like in the Bullfighting, Goya painted them exquisitely . But when the horses performed a marginal function in a composition, for example in the Picture of Fernando VII, they assumed a clumsy, inanimate quality.
Read the entire article (in Spanish) at abc.es
January 29, 2009: The UK Daily Mail reports on the ongoing authorship debates for Colosso here.
"Colossos" by an assistant, claims Prado
June 27, 2008: The long-running argument over 'Colossos' (also often titled "The Giant") has been going on since at least 2001 when Goya scholar Juliet Wilson-Bareau stated the painting was probably by someone else. Research on this has reached the point that the Prado is virtually giving this opinion "official" status:
Danny Wood at the BBC online wrote:
The Colossus, painted early in the 19th Century, is traditionally attributed to Francisco Jose de Goya.
But museum experts now say that an investigation has revealed new information about the possible artist.
The revelations follow the identification of two initials - AJ - at the bottom of the canvas.
According to the Prado's investigators, those initials link the work to Asensio Juli, a friend and collaborator of Goya.
The painting depicts a giant walking through a landscape as people flee in panic.
The style of the Colossus was thought to be connected to paintings by the artist about Spain's war of independence against the French.
But, on the basis of their ongoing investigation, the Prado's specialists say they refute the view that Goya is the painter. 
The identification of Arsenio Juli seems to answer the caution given by Goya writer Sarah Symmons in our interview from 2006:
The question with Goya is that if he didn't paint works like The Colossus, the New York Majas, or the Black Paintings, then we have to find a new artist as their author. If there were such an anonymous artist who for various reasons wanted to keep their identity a secret, then it's a challenge to find out who they were. I know some people have suggested Javier Goya, but I don't believe that.
The UK Telegraph has more on the Prado announcement, written by Elisabeth Nash from Madrid:
... in an announcement set to raise a storm in the art world, the museum said yesterday that the celebrated El Coloso was not by the Spanish master after all, and was probably painted by a pupil in his studio.
In a devastating critique, the museum's chief Goya specialist said the painting, made during Napoleon's occupation of Spain after 1808 and long seen as one of the artist's most dramatic portrayals of the horrors of war, was "a pastiche".
"Stylistically, it is completely alien to Goya," said Manuela Mena, the Prado's senior Goya specialist who has studied El Coloso and doubts over its attribution for nearly 20 years. She also revealed doubts over at least three other Goyas held by the Prado.
..."The person who painted the bulls in El Coloso knew nothing about the anatomy of a bull – which Goya knew everything about," Ms Mena said. "The donkey looks like a furry toy, nothing like Goya's perfectly executed donkeys of the same period. None of the details correspond to the Goya we know."
The UK Independent also chimed in with a headline saying "Is Goya masterpiece just a colossal mistake?" They cover the ongoing debacle. Article quotes respected Goya authoirty Julie Wilson-Barreau, who says:
Doubts were first cast on El Coloso's authenticity – and that of another celebrated Goya work, La Lechera de Burdeos (The milkmaid of Bordeaux) – by the British Goya specialist Juliet Wilson-Barreau, a member of the scientific committee that organised the Prado's new exhibition.
"The works lack energy and a good display within the framework of the painting, faults that are completely abnormal in the trajectory of Goya," Ms Wilson-Barreau wrote in Spain's art review El Periodico del Arte in April 2001. Giving El Coloso the definitive thumbs down, she added: "Almost all the specialists are in agreement that it is not by Goya." Ms Wilson-Barreau's doubts were shared by Manuela Mena, the Prado's senior Goya expert and curator of the show which opens tomorrow.
However, they were furiously dismissed at the time by the Prado's then director, Fernando Checa, who insisted both works were authentic. A year later, Nigel Glendinning, professor of art history at London University, wrote an academic study robustly defending the painting's authenticity. Almost nothing further was heard about the disputed El Coloso until this week, when it emerged that the canvas was excluded from the forthcoming celebration of Goya's war paintings.
On
April
4,
2001,
a
news
article
translating
some
of
the
comments
of
British
art
historian
Juliet
Wilson-Bareau
in
the
Spanish language The
Art
Newspaper, disputed
Goya's
authorship
of
this
painting.
A
Prado
representative
was
reported
to
acknowledge
the
report
as
probably
true.
An
other 150
Goya
images
were
also
called
into
dispute.
Thus
far,
the
hard
evidence,
or
proof
for
these
assertions,
seems
to
reside chiefly
on
the
opinions
of
these
experts.
Whether
counter
opinions
will
be
presented,
or
if
there
is
indeed
something more
tangible
supporting
these
claims,
to
my
knowledge,
has
yet
to
appear.
-
Erik)
A
screenshot
of
the
news
at
cnn.com
:

(To
read
a
defense
of
the
authorship
of
this
painting
as
belonging
to
Goya,
read
the
material
we
have
on
Professor
Antonio
Pereles here.