First: all demons in the bible are exactly like the one you described, so you can't guess his name. Demons appear in visions and look similar to the images of the angels you see every day. None of them bears horns, they just look like huge humans. Lucifuge Rofocale, as an example, appears as a bald man, 2.40 - 2.50 meters tall, wearing a robe of bright shining silver with gold highlights. He is rather quiet, but when he speaks, he has a deep voice with a weird accent. That was no demon you've seen, it was perhaps the baphomet, the symbol of witchcraft.

You need to get over it and as soon as it enters your mind, change your thoughts to something positive.

EDIT: Those are not demons, I would call them malevolent spirits or devils.

As long as you identify and give it life, it will have life.  Try to change from within yourself first, then the demon can be eliminated in thought as well as deed.  

Going into exercises to identify the demon gives it life.  It also gives it the energy of your attention.  Do not feed the trolls.