The source of the modular decomposition graph
Upon the suggestion of Louis Esperet, I contacted Philippe Gambette  and Christophe Paul, who confirmed promptly. Paul designed this graph for his Habilitation thesis. When they created a Wikipedia page for modular decomposition, they used this graph. Maybe it's the beginning of its wide adaption. It's also featured in the well known survey of Michel Habib and Christophe Paul (DOI:10.1016/j.cosrev.2010.01.001).

Some nice properties of this graph include:


it is a permutation graph
its prime node (the bull) is a prime graph containing a vertex that avoids every induced P4. When such a node exists, it is unique.