That giant monitor is a pretty nice steed, I reckon. It has very sharp claws and a well developed musculature that must be efficacious for defense.
It is, but kvaksas try to avoid using them on combat. They are usually bred in appropriate Quarlvák cloisters, and the friars try to repress their primal hunter tendencies as much as possible. They found out, should these lizards be hand-reared by greens from birth, never experiencing the taste of blood, they become quite benign, more so over multiple generations. I imagine them having some seaborne aggressive iguana-like ancestor, quite likely still living on their native islands.
Monitors brought up in this way are then ready to serve as pack animals, and for plowing, mounts also but hounds are preferred for more swift travel without burden. They are said to bear form of the Veláka Koahrr, who shaped the world, sculpting out mountains and valleys, and these mirror it by moving the plough and plowing the earth, it is understood as their mythological purpose.
The reason they try to keep them peaceful is, they are more instinct driven and could easily go on a rampage when distressed. And they are indeed ferocious in battle, and when ticked off do not recognize friend from enemy, unlike hounds. Though, even if such scenario was to occur, the beast would be likely re-usable if successfully calmed down. A bad tempered hound on the other hand is extremely dangerous to a township and needs to be put to death forthwith.
When it bites its victims, can it envenom them with an anticoagulant poison, like a Komodo dragon?
Although quite obviously modeled based on Komodo dragos, these do not possess any kind of venom.
And what are carrying these respectable frog merchants? Ginger? Other spices? Bast ropes maybe?
Is it also a dead rabbit that I see close to the rider? It looks very much like that...
Not sure if ginger is harvested in kvaksa lands, some spices perhaps. Ropes definitely, as well as straps and leather, utility of these basic objects is immense. They could have a dead rabbit in there, too, they do live on the great island. I think it was supposed to be some straps and cloth, but I can't remember
Other than that, there could be honey among the wares, or just empty pots, although those are quite easy to make and each village should have a potter of its own. The idea behind these merchants is, on Báksev and the great island, once multiple colonies were established, one's trade could be visiting each and making coin. Such trader would have a base in one of these settlements, usually their family house, which could also produce some wares if it was a family of craftsmen, from there they would embark on their way, selling and buying at each stop, and returning to home. There wasn't any common currency in this period, so haggling and exchange would be the procedures. Materials valued simply for their beauty, such as pearls or gold would be preferred, but other basic materials, or even food could be traded. A monitor or even multiple of them packed with wares would be a must-have, and the trader would often take some company with him, should anything unexpected happen.
Besides, I always imagined your frogs moving by hopping, but I realize now that it's probably more convenient and look less awkward in a quadrupedal manner!
They do jump also. In combat, especially with a much bigger opponent, the ability to spring swiftly is vital and the tactics to fight foes like humans or large beasts are often based around it. Scouts and messengers would also rather jump than crawl, although that's a given when trying to travel quickly. In a social setting, however, jumping is considered quite rude, and one is to crawl.