One of my species called the Pinnokiins has a nose-like horn on their face. This horn also works as their equivalent of nipples. There is a milk gland connected to it, at least during the time when they use it to suckle their young. Would it technically still be a horn?
Narwhal's horn is actually a tooth.
Goat horns are bones with keratin, like nail and hair, on top.
Rhino's horn is all keratin with no bone underneath.
So people call a horn whatever is sticking forward or up from the creature's head.
Problem with your design is that the offspring are small, and they cant reach high. So mammary glands are located close to the ground, so that the offspring can reach it. Horns are high up and are unprotected from the wind, sun, predators, which are all harsh conditions for the offspring.
It is of course called a milkhorn. The scientific name is cornu lacteum. When removed after death of the animal, milkhorns look like this. They are calciferous structures made of a substance similar to the shells of mollusks.
Due to the fact that the Pinnokiins are an endangered species, these horns are now very rare and expensive. In fact they are endangered precisely because people want these horns. Firstly their pure white appearance makes for a wonderful ornament. Also, in some cultures, they are used by sentient creatures for pleasurable activities that I shall not describe here.
In our everyday world of Earth, the nearest thing we have in appearance is the natural white clam shell as shown below.
Well it depends if the structure's core is made out of bone. If the structure does have core made of bone then it is a horn, even if it has a mammary gland. If there is no bony core then the closest thing it would be is a really specialized nail.(I am going by the dictionary definition of a horn)