It's not a mammal but gives milk to its children.

The Toxeus magnus is a jumping spider, a group generally known for being loners. But they apparently live in family groups, where the mothers care for their young — another rarity among spiders. The spiderlings stay in their home nests for at least three weeks after hatching. Here, they get well fed, i.e. milked.

Their mother secretes a liquid very much like milk from her epigastric furrow, a fold on her underside that she also uses to lay eggs. For the first week, she dabs the droplets onto the walls of her nest, and the youngsters scurry over to suck these up. After that, they drink from the furrow directly. During their crucial early period, the spiderlings rely on the milk as their only source of sustenance. It doesn’t have a lot of fat or sugar, but it’s loaded with proteins—four times as much as the equivalent amount of cow milk.