Mortus Horns are acquired by cutting up Mortus Lungfish caught through Fishing. Go to Fisher Hai-Luk in Cetus, select the desired amount of Mortus Lungfish available, and select the "Cut Bait" option to extract the components. Each Mortus Lungfish cut yields one Mortus Horn.
Alternatively, Fisher Hai-Luk can sell Mortus Horns in bundles of 10 among her rotating stock of "Daily Specials", which can be bought for Platinum. Appear on night time in pond side.