They are quite rare, you can use them to fight against alpha mosas if high level and strong.
You can tame them with cooked prime meat, but you need 30 cooked prime meat in one stack, if you have 10x taming boost, that would be no problem, if not, you might need 90 cooked prime meats if your on easy difficulty, alpha mosas have 100 black Pearl's in their inventory if killed, be careful when finding a tuso, because they can act like oily rocks in the sea floor, they do not have surface limit, which means they can chase you forever and they are fast, if you have about 5 taming space, you can use carbonemys as a bait for the tuso, without a bait, below the Tuso's name it will say "doesnt want to be tame right now, that sounds impossible to tame, that's because you cant tame it without a bait. To make a bait, you need at least a couple of carbonemys, about 4-3 carbonemys, then make sure you have cooked prime meat (a lot of cooked prime meats) got the tuso and get of your tame, let it grab.
After the tuso grabs the carbonemys, it will say "feed it cooked prime meat!" Whenever you have a 10x taming boost, it will say "Put a soothing balm to tame faster!", you get the soothing balm in the cave of the hunter, in other words the southern cavern (cave), to find the southern cave, go to the south, somewhere in the middle of the south beach, find a hill in the southern beach in the island, there's a hill with small amount of metals waiting there, and also small amount of Crystal's are located in that grassy hill, once you find that hill, there should be a stone structure that cannot be destroyed, once you find it, look on the southern beach infront of you, then look at your left, there should be a river connected that leads to the swamp, go the river and if you find a canyon, or and ally way, go through it, and stop in the middle, look at the walls of the canyon if you see a weird opening or a strange opening, go inside it.