Dear Martin, 

the natural innate response is fear of snakes (Ophidiophobia). It is more normal to fear than  not to fear snakes. Even apes do. This we were taught already in our basic psychology books. 

Article How specific is specific phobia? Different neural response p...

Article Therapists' and patients' stress responses during graduated ...

This question is really very interesting. I also wish to know the measurement of fear. I have found the followings which may be helpful in this regard.

http://www.sciencedirect.com/science/article/pii/0005796765900409

http://www.polygon.com/2014/10/29/7081075/horror-games-testing-until-dawn-amnesia

http://bjc.oxfordjournals.org/content/37/4/658.abstract

http://bjc.oxfordjournals.org/content/37/4/658.full.pdf

 I have got some more information about fear measurement.

http://www.ncbi.nlm.nih.gov/pubmed/15586383

https://www.researchgate.net/post/Is_there_an_established_scale_to_measure_fear_of_crime

http://garfield.library.upenn.edu/classics1980/A1980KD94200001.pdf

Article The Development of a Scale to Measure Fear

In the old days clinicians generally developed ad hoc scales. you could describe a series of situations for example with snakes looking at size, distance, dangerousness, protective factors and so on. Would you fear a poisonous snake in your bed? Would you fear a small snake seen through a window etc. the point of the scales would be to work out a system of desensitization and to measure progress.

Distinguishing rational from irrational fear is not always clear cut but may be part of the scale.

Other scales take the form of a fear survey eg heights, spiders, snakes, but the field is vast just google phobias.

Thank you all for your links and hints! I just developed some items on fear of height, because it seems to be no such scale in German language (although in english).

Measureing fear of snake I found the SNAQ.