Some of the new scenario monsters tend to be unfriendly towards tamers and their pets. Some of the 3D monsters are also something to be weary of.
These are certainly an oddity. My personal experience with them was not particularly dangerous, since I was hunting them with a grandmaster bard/tamer. When the serpentine gets to critical levels, they will cause dragons or white wyrms to turn on their owner. Many people have had cases where their pet went wild while hunting the serpentine (this causes bonded pets to go wild as well!).
Not only do these summon undead, but they can make a pet turn on its owner or cause it to go wild, just like the serpentine dragon does (this causes bonded pets to go wild as well!).
These are certainly tough monsters in any light. Pets cannot be commanded to attack golems directly (they refuse to attack), but will still fight if you are attacked while guarded or the golem attacks the pet itself. If you watch closely, the pet will stop attacking the golem occasionally, even with the golem still smashing on it. It seems as though the colossal blow effects them. Golems appear to have some sort of reactive armor, causing strong pets to become a danger to themselves. The ideal pet to pit against a golem is a unicorn or ki-rin.
The warriors and mages aren't too much of a problem for a dragon to handle. However, the jukan lords do have an interesting quirk to them. They can peacemake pets (I have only seen this happen if other players were fighting the lord at the same time as the pet). The pet can be commanded to attack the lord again, though. Jukan mages will cast beneficial spells on the jukan lords (I watched several lords go from nearly dead to full health in a matter of seconds). Jukan lords also bandage themselves, sometimes wield bows of dragon slaying, and will pull out a katana to use once they run out of arrows.
Overall, they just take a long time to kill. They don't dish out a whole lot of damage, and take very little in return. What makes them dangerous is the poisonous they spray out. It is deadly poison and hits all targets within a 2 tile radius, making it dangerous for veterinarians.
As with all meer, the meer mages have a strong afinity towards nature, particularly with beasts of nature. They have the ability to command one's pets to revolt against their owner and cause them to go wild (this causes bonded pets to go wild as well!).
For the most part, the dragoons aren't that big of a deal. They cast a little magic, but their melee isn't too bad. However, they do have a special attack that looks like a poison green explosion (2d client) that causes the pet to lose 1/4 of its health. This is an area effect attack, similar to mass curse, but with a 1 tile radius. If you stand with at least 1 tile separating you and the dragoon, you will have little trouble taking him out to get the swamp dragon. Anything roughly the power of a unicorn/ki-rin or stronger should have little trouble killing the dragoon.
When attacked with magic, the plague beast will hurl green bile at its attacker, which does a fair bit of damage. The plague beast's melee damage isn't all that bad. If you are using a magical pet, it is recommended that they be out of mana before tangling with the beast. While fighting against a plague beast, "plague spawn" will form off of its body. If ignored, the plague spawn will eventually "disolve" and die.
As far as damage output goes, solen warriors are pretty weak. However, their big danger is from the acid they spit out. After a certain amount of damage is done to the warrior, you will receive a message that its acid sack as been broken. Once that happens, it will spit out green blobs of acid onto the ground. Usually, it goes out in a 1 tile radius from the warrior, but can go 2 tiles away. If the acid is spit 2 tiles away, it will hit anyone attempting to use veterinary on the pet. If anyone or anything is standing in the pools of acid, a hissing sound will be heard and anything standing in the acid will rapidly lose health. If this happens to a pet, simply call them out of the pools. The acid will go away after a short period of time.