What animals are you afraid of ?

[Maciamo]

That's because the reason most probably isn't instinct. It's more the (social) environment you grow up in & personal experience in your early childhood which influence subconsciousness.

Although phobia can be partly acquired, researches have shown that instinct played an important role. If it was only environmental, siblings wouldn't be so clearly different, even opposite, in their fears, and the fear would be controllable using one's cerebral cortex (reason, self-control...). But it isn't.

It is apparently the result of millions of years of evolution. That is why, even rational people who know perfectly well that a big spider or snake is harmless have a severely increased heartbeat, sweat and stress level when they face one of them, an even more if it comes close to them or touch them. I am not sure if you have any such phobia, but I have tried to control my emotions about spiders and the best I could do is stay reasonably calm when I was at least 2m away from a giant spider (Australia was a good trial, as I was confronted more than 10 times to such giant spiders).

From the BBC on Zoophobia

Phobias may be learnt from another person. For example, a child may see his mother screaming and reacting hysterically to a spider, and from then on associate it with danger. A traumatic experience may be the trigger: a child may have been viciously attacked by a dog, thus developing the phobia. Books and television are also responsible - there are many people who are still afraid to go into the water after the film Jaws.

Inate biological instincts may also be partly to blame. Primitive humans would have needed to avoid certain species of animals, as we do now, such as venomous snakes, poisonous frogs, tarantulas and wolves. These instincts may simply be particularly strong in some people.