There does not seem to be a horn on Cryptids Island. There is a flute (a pennywhistle) which you receive after the submarine ride, after you defeat the sore loser at darts again.
If you use the sub, I think you will find/see? Nessie.
Along the bottom, you will find a brightly-colored Nessie face and get a photo of it. Bring it back to the pub where it is revealed to be a movie prop.
You don't. When you are in the submarine and find the Nessie "head", position the light on it and take a photo. Show it to the dart players at the pub.
The Pub is to the right at Loch Ness. You can get tickets for the rowboat and submarine rides by beating the dart players there. You will also find the matches you need for the lantern you get in the Himalayas.
You get the submarine ticket from the other two players in the pub, by winning at darts. Take the picture from the submarine back to the pub, where you find it is part of a movie prop. But the first dart player angrily returns and wants a rematch. Beat him to get the pennywhistle (from the bartender, not the sore loser).
In the monastery.
You find it caught on the fence of the farmer's brother.
Yes, you can find proof of The Jersey Devil, Bigfoot, Nessie, and the Chupacabra. (see related question)
Dive to the bottom and then go left.
after you cut it it automatically goes into your inventory.
There are some matches in the pub at Loch Ness.