They could easily be more of a detriment than a benefit
To prove this point, let's assume the best possible scenario for the materiel properties on that unicorn. We will say that it is hard and sharp enough to punch through armor and that the unicorn's skull and neck are properly reinforced to survive an impact at full charge.
Now let's compare that horn to ...
During the London Bridge terrorist attack, a narwhal tusk was taken from a display case and used as a weapon against the terrorist. This is pretty similar to a Unicorn horn.
The mythological descriptions of unicorns vary somewhat between two different, yet similar themes. The similarities are that in both depictions, unicorns have a gracile, horse-like body plan with a roughly one to two metre long ivory horn projecting perpendicular to the head from between and just above the eyes. The differences are that some legends depict ...
A horse is not designed to charge head-first and head-down into things, regardless of whether the horse has a horn or not.
The horn will break or the unicorn will snap its own neck or both.
Even if the unicorn survives the initial impact he/she will at least be groggy or dazed from the blow and easily dealt with by the opposition.
It's a one-...
The osmeterium is a defensive organ found in all papilionid larvae, in
all stages.1 The organ is situated in the prothoracic segment and
can be everted when the larva feels threatened. The everted organ
resembles a fleshy forked tongue (not unlike a snake tongue), and this
along with the large eye-like spots on the body might be used to