Serpentflesh is actually a bit of a misnomer when it comes to this particular type of golem; it isn't made exclusively from snakes. Rather, the serpentflesh golem is actually made up of any body part that belonged to any of the serpent reptilian races, which can include kobolds, lizardmen, yuan-ti, nagas, and similar races.
Essentially, a serpentflesh golem is almost exactly like the ordinary flesh golem (i.e. the Frankenstein's monster): it's a mindless construct immune to many forms of magic, save for those that deal certain elemental magic. However, due to the serpentflesh golem's inherent bodyparts, it's also venomouse, capable of delivering poison through any snake-headed section of its body.
The serpentflesh golem is meant as an insult to the scaled races. Most of the scaly ones are too proud to create abominations from their own flesh, and seeing the non-scaled races create these abominations makes their cold blood boil.
Also, animation because animations are fun.