There has been talk of having the Tarrasque in a desert, the only ready available food being its flesh. They only drink its blood. But what if the area only started out as a desert? The first watchers, when they ran out of food and drink, might have taken to desperately using the Tarrasque for sustainence before discovering that it is surprisingly tasty, even a bit addictive, when prepared properly...
Time went on, and they figured out how to make many sorts of food and drink from the Tarrasque's flesh. Fruit and vegetables were still a commodity, of course, because the tongue eventually desired something else, but there was little need to find any other source of food. The increased desire for flesh, however, had left the beast bleeding far more than previously. Hasty ditches were dug to drain away the red liquid, and a few enterprising guardians decided to see if they could grow grain in this arid place by watering it with Tarrasque blood. The result was... Unusual. But tasty, and almost as addictive as the flesh itself.
Later, as the population began to grow and it was impossible for any more flesh to be removed - The bones and organs of the Tarrasque being constantly exposed now by cutting, and even those did not remain completely safe - blood drains were installed at a great cost of life beneath the beast by hired dwarves who dug in from below. An elaborate pumping and irrigation system was set up to ferry the blood far and wide to the fields of what was now becoming an empire. Where there was once a forest red grain blew faintly in the breeze, and the red corn was traded far and wide by their farmers.
None needed to go hungry, or thirsty, the blood waters the crops... And there are fountains that all can drink from. The skin and bone of the beast provide powerful magical weapons and armor for their defense... Its organs were found to be even more nutritious than its flesh, though far too useful in magic to be eaten by any but the richest among its people. The crops do well as an export, and no one seems to find it unusual that the people of the Tarrasque empire have eyes just a little more slitted, teeth just a little sharper, nails just a little blacker.
The blood, the flesh, affects everyone differently. Most live longer, some heal faster, some become stronger, some even become little more than tiny versions of the Tarrasque; monsters that are quickly put down by the guard. But that is mostly in the city, in the countryside they just eat the crops... And the changes are mostly cosmetic. They are still friendly, helpful, farmers living perfectly ordinary lives. At least to those outsiders who look in on them. They couldn't know about the blood sports in the fields at night. How those who don't change are killed... About the occasional monster born by ripping its way through a farm wives stomach before the rest kill and it it... and the mother... something that is said to be a good omen for the next harvest. They could not know about the high birth rate, and how it is balanced out by the cannibalism. As far as the outside world knows they are just farmers.
And they want to sell you some fruit... Don't mind the apples, its perfectly ordinary for them to be red all the way through.