For the tutorial with thumbnails of how the centaur was constructed, 
see the Classic-Castle forums:
http://www.classic-castle.com/forum/viewtopic.php?f=14&t=18234

I've been experimenting with making a custom centaur using the body 
of a Lego horse. I figured I'd share the process with the forum.

Meet the centaurs, Abusson* the Vile and Ouranon* the Valiant:

abusson.and.ouranon.jpg

First I had to pry open the horse halves. Then I removed the horse 
head and carefully cut out the front posts (which serve to attach the
horse head). One must be careful not to "force cut" the posts to 
remove them but to either use a small fine-tooth saw or whittle away 
a little bit at a time using a sharp knife. Otherwise you run the risk 
of cracking the horse body.

explodedview.jpg
horse.neck.inside.removed.jpg

I then used two curved slopes and a plate to "fill in" the hole left 
by excavating the horse head and front posts, and then attached an
"old" torso to the 1x1 plate. The "old torso" is necessary because 
it does not block the stud like the newer torsos do. One can "modify"
newer torsos to accept a stud.

new.insides.friction.fit.jpg

The width of the curved slopes allows for a "friction fit" that holds 
the two slopes together while creating a "curved" transition from
the minifig torso to the horse body. I first superglued the two 
curved slopes together, but found that it was not necessary to do so
because the slightly tight fit kept the two slopes together just fine. 
The friction fit also allows for the centaur to be posed in a
"rampant" position because one can move the new "neck" forward. Note 
that the torso itself can pivot because it rests on the 1x1 plate's
stud. One can remove the 1x1 plate so that the torso fits closer to 
the horse body, but that removes a bit of "pose-ability".

nakedrampant.jpg

When putting on barding, some changes needed to be made. I basically 
"built up" the area that "filled in" the hole so that I could put the
curved slope in the front to close the gap in the front of the barding. 
The addition of the front curved slope reduces the 'mobility' of
the centaur's trunk (formerly the horse's neck).

alt.insides.with.barding.construction.jpg
alt.insides.with.barding.jpg

Overall, I was happy with the results. The finished product looks 
relatively clean, and the curves of the horse body fit relatively well
with the curved slopes.

naked.jpg

*Abusson is a form of the Greek word from which we get the term 'abyss'. 
Abusson means "to the abyss." Ouranon is a form of the Greek word for 
'heaven'. Ouranon means "to heaven."