How to Unlock Ghost Guide
In order to unlock the Ghost monster you'll first need to have a monster on your Ranch pass away either through old age or injured badly in a tournament. Once you lose a monster Colt's Master will show up and ask you if you'd like to hold a funeral ceremony and build an altar for your lost companion. Tell him yes, you would like to build an altar for him.
After a few months go by Colt will ask you to upgrade the altar two times; the second time around Colt will find a Stick while cleaning the Altar. You're told that this Stick is a gift from the heavens and that you should take it to the Lab and try using it as a secret ingredient.
That's all there is to it! Using the Stick as a secret ingredient during any combination of monsters will net you the Ghost.
Ghosts are one of the most powerful Intelligence based monsters you can get. They live on average for 8y 4mo and they have high stat gains in Intelligence, Skill and Speed but very low stat gains in Life, Strength and Defense. In addition to this, they also have a +1 bonus towards Speed which will help you boost that stat quicker than usual.
Since Ghosts have low stat gains for both Life and Defense they're best designed to be paper cannons which focus on heavy hits with high Intelligence and dodging all attacks with high speed. Ghosts also have very quick Guts regen during Battle; all of these things make them one of my favorite monsters for someone just starting out.