Not Bigfoot's footprint, but possibly that of the Yeti (a similarly described creature). It is above the monastery on a trail to the upper left. You need a photo of the footprint in the snow.
In the monastery.
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.
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)
There are some matches in the pub at Loch Ness.
Find all 4 cryptids
after you cut it it automatically goes into your inventory.
Dive to the bottom and then go left.
first you have to find all the other cryptids then you go in the helicoptor to the new location and follow him to his cave
The prize for spotting Nessie during the Cryptids Island preview (beginning December 2, 2010) was a Nessie Hat. (see related link)
You find him in the lab, where you first talked to him. From the helipad. it is just to the left.
If you use the sub, I think you will find/see? Nessie.