Alternatively, it could be a prized souvenir from a pet turtle he used to cherish, and perhaps the death of which prompted him to assume the Turtle Master title and attempt to wreak vengeance. I think it's beyond what can be deciphered from this piece how he came to pair with and harness this creature, but as I say, I'm of the opinion that the big shell was attached on by the Master to his steed, and that it was happenstance that this then also provided a smooth yet solid saddle for him.
"Behold the turtle. He makes progress only when he sticks his neck out." Korean Proverb
We're intruders in some sort of lush, mountainous valley, the fence on the left seems overgrown and unlikely to prevent any entry or passage by the most inoffensive of souls, let alone provide defence against this amphibious marauder.
I'm not convinced that the batch of tiny turtle/terrapins are fleeing the Master (his back-pack looks like it could be filled with shells of previously snared animals of their ilk), although that could be what's going on. I think it's slightly more likely that the depiction is of them as part of a horde or army, rolling and sliding ever forward to capture new pastures for the Turtle Master to claim as his realm.
He's wearing a very nice hat, slightly too short trousers (possibly tactically selected, as you'd imagine that grass gets quite dewy), and a kind-of rusty orange pull-over. I can't imagine that this is his full Turtle Master uniform, if so it's mightily underwhelming, so another possibility is that the protrusion from his ruc-sac is part of the garb he puts on when he goes into action. My deduction would be that this depicts the troupe en-route to a later appointment.
"Looking for peace is like looking for a turtle with a moustache: You won't be able to find it. But when your heart is ready, peace will come looking for you." Ajahn Chah
The ogre-ish animal he is straddling wears a semi-clownish expression; a miss-leading Land Before Time style goofiness will likely have lured in and snared unsuspecting foes before now.
He seems to have been fitted with a very standard reign-like contraption to the mouth by the Master, the apparent flimsiness of which leads me to assume that he is at least partially tame and in league with his rider – not being controlled against his will. But he does look a touch dozy and easily distractible, so it makes sense for the Master to have the option of reminding periodically him who's boss.
"The turtle lives 'twixt plated decks Which practically conceal its sex. I think it clever of the turtle In such a fix to be so fertile." Ogden Nash
The Master is smoking a very thin, threadlike pipe. Nothing in the picture explains what substance he is enjoying – if you take the position that he is in fact in pursuit of the flock of miniature beings, to feast upon their fleshy innards, scrape clean their shells , add them to his haul and continue to conduct a purge of all turtle-like things on the continent, then it would not be too much of a leap to imagine him grinding the bones of his unfortunate prey into a fine 'snuff' and greedily gulping in their fumes to "feel at one with his quarry".
It's a striking vision certainly, but as earlier stated, not one that I necessarily subscribe to.
An added ambiguity is the extremely dainty little cow-bell-esque device he's brandishing – a petite, fancy embellishment at odds with his no-frills, man-with-a-job-to-do demeanour. One interpretation is that this is a freshly secured trophy: he and his troupe have just vanquished the previous occupants of this desolate domain and the bell-ended baton pried from the erstwhile ruler of the land. Another is that he just has a soft-spot for pretty sticks that make a jaunty little jingle-jangle as he rides along.
"Rise up, my love, my fair one, and come away. For, lo, the winter is past, the rain is over and gone; The flowers appear on the earth; and the voice of the turtle is heard in our land." The Bible
A lush, beguiling work which demands the viewer use their imagination at every stage.