Ah, the chameleons in the world of rock music. Chameleons in the best sense preempt environment changes. If the winds of music are going one way, the chameleon was, in all likelihood, there first. But they ceaselessly reinvent themselves to the point where to try to describe the artist in one word, or even a sentence (or a paragraph, or so on and so forth...) is a fruitless exercise. There are only a couple of chameleons in rock music that are of note, and I think it's necessary to at least examine each chameleon in some sort of detail or length. These two are Bob Dylan and David Bowie, and they both have led stellar careers where each milestone coincides with some sort of reinvention and/or landmark work that either pioneered a genre or proved to be that genre's finest work.
But really, which image would best describe "Bob Dylan" as the man? You'd be hard-pressed to pick one picture and say "That there is everything Bob Dylan was."
He was a folk revivalist, a neo-folkie who brought the genre back into the popular consciousness pushed its boundaries in form. He then forsook folk for more fertile territories by going electric, infusing the ferocity of rock with a lyrical inventiveness that has never been replicated since. But then he became "the lonesome hobo" and then a country crooner. And then, in the wake of his marriage, became the epitome of lovesick and in one fell swoop created the "confessional singer-songwriter" genre, all while penning a classic album that simply is the best breakup album of all time, if not standing tall as one of the greatest records, ever. Next he found himself and became born again. That didn't last, though, as he changed his colors and became the world-weary, grizzled old wise man that he is today.
In every sense, he either invented the genre (neo-folk), or proved to produce the finest records in whatever genre he happened to be in at the time (Highway 61 Revisited, Blonde On Blonde, among the rest of them). Perhaps you could make an argument that in strict terms of music, you'd be hard-pressed to find a genre that Bobby D actually invented for himself. But I think that there's almost no such thing as a completely "new" genre: it's built from the blocks of older genres and appropriated and given a fresh new light, which is what Bob Dylan did.
Regardless, just by looking through the previous paragraph that condenses a 40 or so year career into a paragraph probably does Bob Dylan oodles of grave injustices. No man has ever been as slippery as Bob Dylan. Hard to get a hold of, because he is, and hard to get a hold of because he has changed too many times to account, to keep up with the times as they changed around him.\
Now, I could have also picked from a ton of possible photos for David Bowie, but of all the ones I could choose, I prefer this photo, as I wanted to avoid using the album cover for Low at every turn (because really, I would have no problem with that). But David Bowie has led almost as long as a career as Bob Dylan, with almost as many twists and turns, which were arguably more drastic than Dylan's.
First plumbing psychadelic folk, Bowie later turned to glam rock, which later became a brief foray into soul and R&B before he went entirely experimental and avant-garde with the so-called "Berlin Trilogy," a classic landmark. After "retreating" a bit into more accessible music for a long while, Bowie turned to electronica before basically spending his time reinventing his legacy.
While at first it doesn't seem like as many twists and turns as Dylan's career was, Bowie's were undoubtedly more revolutionary. Bowie essentially invented glam rock with Hunky Dory and the Ziggy Stardust record. He also pioneered the use of electronics in music and essentially helped form post-punk and New Wave with his "Berlin Trilogy." Those achievements alone are astounding, but when taken in the context that it was all basically done by one man, who had either the wits or just the fleeting sense of creativity to change his musical appearance so drastically, then this proposition becomes mind-blowing. Most artists spend their lives daydreaming about inventing genres and becoming a pervasive influence in music; David Bowie spent most of his time, well, inventing genres and becoming a pervasive influence in music.
And so, there you have it. They're a little brief, but I hope you get the gist of it. Dylan and Bowie were both quintessential chameleons: they never remained in a state of stasis for very long in their careers, as either they just kept on changing to their environment or actually creating a new environment around them. Because of their successes in their changes, they both have had lasting influences in rock music and have both created, together, at least 50 records that must be heard before a person dies. That's pretty damn impressive, right?