Which type of animal is most similar to rodents?

Mammals usually are taken to mean placental mammals - animals which have hair, warm blood, bear living young and suckle their young. Other more primitive animals are still included in the category of mammal, monotremes like the echidna and platypus which lay eggs and the marsupials which bear live but very immature young which must live in special pouch in the mother's body.

There are no living examples of the therapsids (very early mammal like dinosaurs) alive at present.