Ruminants and Monogastrics

Yes. Hind-gut fermentors are psuedo-ruminants.

Cattle, sheep, goats, deer, moose, all species of antelope, giraffes, camels, llamas, rhinocerouses, yaks, bison, buffalo, elk, reindeer, caribou, and muskox are some of many species that are all ruminants (or in your case, not non-ruminant or monogastric species.)

Because their rumen isn't fully developed. Calves are born as monogastrics, with the other three chambers as only sacs of useless skin along with their abomasum or their true stomach, which resembles the stomach of all other omnivores and carnivores. The rumen doesn't start to begin to develop until a week after the calf is born; the rumen completes development by the time the calf's 3 to 4 months old. So when calves suckle the milk from their mothers, the milk is directed straight to their true stomach, which is designed to use and break down the proteins, amino acids and nutrients from the milk (containing animal proteins!) for the calf's growth and development. The rumen doesn't start developing until the calf starts to consume or "taste test" forages that its mother eats, which is by the time a calf is around a week old.

The Liver.

A rumen fistula is a surgically created entry to the rumen in the left flank of the ruminant, typically permanent and with a rubber or similar material device (a 'cannula') fitted to close it between times of use. It enables the rumen contents to be accessed, and the rumen wall to be viewed and biopsied. Surgically modified ruminants such as these are only found on research farms, where they are used to study rumen function.

Looking in through the fistula, you can see the rumen contents and the rumen wall.

don't you mean ruminants?

Yes I do.

See related question below for such a list.

it helps food digest

The peccary (which is a species of wild pig) is a non-ruminant like all other pigs.