Marsh Tigers are designed for their wet environment. They have webbing between their toes, and "prominent" nostrils placed to make it easier to breath while submerged in water and mud.
Marsh Tigers are so-called because they have stripes along their bodies. The depiction is a Marsh Tiger shows they can be red-orange with black stripes. They have two curved horns on top of their head.
The eggs also have a tiger pattern on their shells, to designate that the eggs are poisonous.
The Marsh Tiger eggs are said to be poisonous. There is no mention if adult Marsh Tigers are also poisonous, or it is a protective trait for just the eggs, as it seems they have no parental protection.
Behaviorally, Marsh Tigers are similar to the franchise dragon species, the Mudraker, in that Marsh Tigers also prefer to wallow in the water and mud of bogs, sometimes submerging up to their noses. This is rather inconvenient if a rider is on their backs when they do this!
Marsh Tigers lay their eggs in basket-shaped nests, that float down streams.
Marsh Tigers rate a "6" on the Disobedience stat. Vikings often use the Marsh Tiger as a 'starter' riding dragon.
Tuffnut Junior was shown to have picked a Marsh Tiger as his riding dragon.
An image of a Marsh Tiger appears in this reference book.