Parental Care
The fact that it is warm blooded, and the ability of female Zebras to nurse their offspring, makes it a mammal.
Yes, a mare is a typical mammal giving milk from it's teats to it's offspring, the foal.
The male and female both protect the offspring but yet the male does not enter the maternal den (breeding den).
It's a mammal trait.
D. Nursing the offspring. This is a general mammal trait, much older than actual primates. Then comes either violence or social grooming, though neither occurs in all primates. Then binocular vision (general primate trait associated with living in trees) and then monogamy, potentially, though that occurs only in a very few species and then not even in the entire species (an example being humans ;)).
marsupials.
They have fur, nursing milk, and birth to live young.
Bats satisfy all of the defining criteria for being a mammal. Bats have fur, female bats lactate and feed their offspring, baby bats are born live (not hatched from eggs), bats have a four chambered heart, and so forth.
Mammal. It has the four things that make it a mammal. Has hair, has a backbone, is warm blooded, and offspring are born alive. If an animal doesn't have these 4 things they are not a mammal.
Mother mammals make milk for their offspring.
Reproduction is animals having offspring. In mammals, this involves sexual reproduction, wherein a male fertilizes the egg of a female, which harbors the new young mammals as embryos. In most mammals, the young are born after much development as placental fetuses, and then are nursed. In marsupials, the young are born at a lower level of development, and continue to grow while nursing in a pouch. In monotremes, or egg-laying mammals, the embryo develops outside the female in an egg, then is nursed after hatching.
Approximately 5-10% of mammal species demonstrate male parental care of offspring, but this behavior can vary widely between species. Males typically provide care in species where offspring require extended care or where resources are limited and need to be shared between parents.