Which animal had or has the longest horn, past or present?

Most horns and tusks are derived from modified teeth.
A 'present' animal with a 'horn' that can reach 3 metres in length is the Narwhal
http://www.enchantedlearning.com/subjects/whales/species/Narwhal.shtml

And the winner is....Palaeoloxodom antiquus germanicus
The longest tusk ever found belonged to the extinct Palaeoloxodom antiquus germanicus, the Straight-tusked Elephant. On average this magnificent animal had tusks 5m or 16.5ft long.
https://www.researchgate.net/profile/Adrian-Lister/publication/264785320_Remains_of_straight-tusked_elephant_Elephas_Palaeoloxodon_antiquus_Falc_and_Caut_1847_ESR-dated_to_oxygen_isotope_Stage_6_from_Grevena_W_Macedonia_Greece/links/53f0e7ee0cf2711e0c431523/Remains-of-straight-tusked-elephant-Elephas-Palaeoloxodon-antiquus-Falc-and-Caut-1847-ESR-dated-to-oxygen-isotope-Stage-6-from-Grevena-W-Macedonia-Greece.pdf

OTHER HORN 'FACTS'
The longest tusk on record is from a Columbian Mammoth tusk found in
Texas which measures 16 feet long (4.8m)and weighs over 200 pounds.
http://www.earthlife.net/mammals/teeth.html

'Elephants, as you might have expected have the largest teeth in the world. An elephants tusks are actually modified incisors.'
The largest tusks of elephant were found in Zaire.
The right tusk - 3,49 metres the left tusk - 3,55 metres. (11ft 5ins - 11ft 7ins)
http://ufdcimages.uflib.ufl.edu/AA/00/02/01/17/00002/AA00020117_00002.pdf

Rhinoceros. The longest horn of rhinoceros (150cms) belonged to female of white rhinoceros which was killed in South Afrika.
https://rhinos.org/blog/how-long-can-a-rhinos-horn-grow/

Marco Polo sheep. The longest horn of this spectacular sheep( photo )is no less than 67 3/8' (1.71m)with a horn base of 16 1/2'.
Currently the World Record Marco Polo sheep shot in 1895 by Field Marshall Earl Roberts with the longest horn measuring 75 inches(1.9m) http://kisima.org.gridhosted.co.uk/q5wa74hj/animals-with-longest-horns-00f16e

NARWHAL whale.Monodon monoceros has a single tooth that projects from its upper jaw and can grow to be 7-10 feet (2-3 m) long. Tusks are usually twisted in a counterclockwise direction and have a hollow interior. http://www.enchantedlearning.com/subjects/whales/species/Narwhal.shtml

Triceratops dinosaur had 3 horns, one on its nose and one over each eye. The eye horns were more than a meter (3 feet%

Fosse4 says:

HAS the longest horns (On average) Bos Taurus Ankole Cattle.
HAD the longest horns Megarceros Giganteus (Giant Deer or Irish Elk) now extinct

Fri Nov 29 14:38:14 CST 2002 (Delete Entry)
(Moved from #24714 - Duplicate question - McG)