Guinea pig

Male guinea pigs reach sexual maturity in 3-5 weeks, females similarly - they can be fertile as early as 4 weeks old.

An excerpt from the article 20 facts about guinea pig

A female can have five litters in a year, although six are theoretically possible. Unlike the offspring of most rodents, guinea pig puppies have well-developed hair, teeth, claws, and partial sight. They are immediately mobile and begin to eat solid food, although they continue to suckle.

Females can become pregnant again within 6-48 hours after giving birth. Pregnancy lasts an average of 63-68 days, and the litter size varies from 1 to 6, with an average of three cubs. A sow can adopt cubs from another female.