[charli (OP)]

Mohammed muses:

"How did horns evolve in mammals and other species and what evolutionary advantage does it give to such animals?"

[charles1948]

Horns are Nature's boxing gloves, evolved so that fights between males can be resolved without a lethal outcome.

[CliffordK]

Horns and antlers are very different.

Horns are made out of Keratin, similar to claws and fingernails. Likely evolved as a claw in the scalp.

Antlers are more related to bone.

Between species, horns and antlers would have evolved as a way to protect themselves.

Within a species, they are often part of a show of strength of males as part of a mating ritual.

[evan_au]

Easier to explain may be tusks - these are enlarged and modified teeth, more often larger in males.

Some are spectacular, like elephants and mammoths
- Some are mysterious - like the arctic narwhal's spiral "unicorn". Scientists have been wondering what they are for (apart from the expected male competition), but some marine biologists recently saw an unexpected use: fishing.

[Bored chemist]

Horns are Nature's boxing gloves

Specifically, they are the spikey sort of boxing gloves...