It is an important question, I'm sure you will agree, as to what qualifies as a 'golem' as distinct from an automaton, an animated statute, and so on. It has long troubled me that the 2nd edition Monstrous Manual treats Frankenstein's monsters, clay golems, doll golems, gargoyle golems, juggernauts and scarecrows as a single category, when to my eye these are an impossibly wide variety of fundamentally different types of thing that are being wrongly brought under the same umbrella. Many a night have I lain awake, tossing and turning in my bedclothes, staring at the shadows on the ceiling and whispering hoarsely to myself about the infelicities of this deep, fundamental misunderstanding and what it suggests to me about the impending dissolution of the world of concepts - this tends to happen when I've been at the whisky. In any event, it's about time I got this off my chest: YOU CANNOT HAVE A MEANINGFUL CAMPAIGN IF STRICT DEFINITIONS ABOUT WHAT IS, AND IS NOT, A GOLEM ARE NOT MADE OUT.
To a certain extent, philology can guide us. According to wikipedia, the word 'golem' appears in Biblical Hebrew once in the Bible, as golmi, meaning 'my light form' or 'my raw material', as in an unfinished, incomplete or uncultivated human being. This has some important corollaries.
The first is that a golem must have a humanoid form. This instantly rules out juggernauts, gargoyles, and so on (though, I would argue, they will also be ruled out for other reasons which I will come to).
The second is that a golem must be fashioned from 'raw material'. This rules out finished goods such as stained glass or steel, and also anything that is or has been alive, such as flesh or wood. It also rules out things like scarecrows and dolls that may have been created for a particular purpose and are now being repurposed as an animated slave/servitor.
The third is that a golem must be unfinished or incomplete in the sense that it is not fitting for some other purpose than being a golem. In other words, something that had an independent existence as, say, a statue, objet d'art, etc., and is now being animated for use as a putative 'golem' is not in fact one.
It follows that there are other categories into which it is more appropriate to put things like juggernauts or scarecrows, to wit:
Animated statues are things like gargoyle 'golems' and stone 'golems' which were originally created for an inanimate, cosmetic/aesthetic purpose and have subsequently been animated.
Automata are things, mechanical or otherwise, that have been 'finished' as putative 'golems' - this would include, for example, juggernauts, steel 'golems', clockwork 'golems', glass 'golems', and so on.
Animated objects are things like scarecrows, doll 'golems', animated chairs and tables, etc., which were created for some original useful purpose but are not deployed as moving servants. Some sages dispute whether perhaps animated statutes and animated objects are in fact two sub-categories of a broader category.
Reanimations are things that were once alive and have now been, well, re-animated - whether as a whole or in a collection of parts. The flesh golem would be the archetypal example, but a golem made from wood would perhaps be another.
It also follows that the only two types of 'golem' listed in the Monstrous Manual that actually qualify as golems are clay, and possibly iron and stone on the proviso that the latter two types must not be statues that have been later animated, but must be crude, unfinished humanoids fashioned from the material in question. It also follows that there may be other varieties of golem that are as yet undiscovered - for example, golems formed from the mud at the bottom of the sea, or from soft metals, or from ambergris perhaps.