Personally, I find it fun to put a little mysticism into my stories. In my daily life, I can't say I am a particularly superstitious person, but I do pay attention to what is around me and enjoy coming across possible symbols. How much weight I give them varies from zero to wait and see.
When I write a book, I am free to give mystical symbols more weight. It takes some research to find the appropriate creature, learn its meaning, be sure it could actually be seen where I need it to appear, and then decide how much it is revealing. Does it come from an outside spirit source or simply the character's need to find something to help?
When I create a trailer, I often put in an animal of some sort that might not have even been in the story but that its meaning suits the feel of the book. For that I have used spiders, hawks, foxes, etc.
In the book itself, using say a dragonfly and having it continually crop up can serve as foreshadowing if there is a character who might know it's meaning.
The above photo was one of a series I took of a dragonfly over at Klamath Lake in 2010 with fireweed in the background. The unusual rounded reflection was pure luck with a sun bubble and the stem as it was not photoshopped. For coloring and purpose, it would be perfect for the book I am writing where the heroine is like the fluffy pink flower, unfocused and uncertain of direction but about to find a metamorphosis coming in her near future.
Dragonflies symbolize change as they spend most of their life under water as a nymph. When they emerge, they fly for only a few months before they die. Seeing them like this is fleeting time they fly with beauty and grace. If a character were to see dragonflies where they did not expect, it would seem a possible indication of major change coming.... if one believed in symbols.
This praying mantis shot, from September 2012, also has a bit of an other earthly feel to it from the big camellia leaf right behind. Being a telephoto shot, the background was lost allowing the insect to have center stage.
This is a female and they sometimes eat the male when they mate. Can't think of a much more apt symbol for a man afraid of relationship or perhaps a man who had bad luck with a wife, she died or left him, and now he worries that the next woman will be the end of him. Perhaps a more positive meaning for the man would be to make sure that the woman he chooses is his equal, that he cannot be overpowered by her, nor would he want to overpower her. Two strong mates and neither will destroy the other.
It's rewarding to think of symbols for a story using something from the animal world be it a bird, fish, mammal, reptile or insect. They can add something to the story and give the writer a little more depth not only for their character but to the events. It can be a small creature or a very large and frightening one.
For life, seeing such things, even better getting a photo adds excitement. In a book, it can help tell the story.