The use of Omens has a very long history as a form of divination and omens can be anything, they can be indicators of both the bad and the good. An omen is a sign, something a person reads into an event that happens, or something that naturaly occurs, and sees it has a foretelling of things to come.
For example, it is said that the Druids would read the flight patterns of birds, often to predict the weather, and many supersitions are really just a form of a bad omen. Like to see a black cat cross your path is said to be bad luck and so forth.
An omen can truly be anything, if a person finds a rock that is formed in a certain shape, or sees two sitcks upon the ground in a particular pattern they could read something to this and take for a sign of something.
There are many different animals that are considered omens, to see certain types of animals can predate different things to come. And if one encounters thier totem animal face to face it is gernealy considered a good sign.