Well for one, the director meant to depict the title of the film "Dogtooth" with a symbol. He could have simply used just an illustration of a canine tooth, if the audience was supposed to take the title of the film literally, but he didn't. Which leads us to the conclusion that there's more meaning to the film than just what the title conveys.
If we take a good look at the symbol, it is in fact a parabola, not a canine tooth. Nevertheless, it serves the purpose of resembling a dogtooth well. But this does not change the fact that it is still a two-dimensional, mirror-symmetrical curve, called "παραβολή (parabolee)" in Greek.
However, the word "παραβολή" in Greek has several meanings. Literally, it is the comparison of two objects placed (=βολή) the one next to the other (=παρά) for the ascertainment of their similarities and differences; a definition that also gives birth to the name of the plane curve, "parabola". Concurrently, "παραβολή" is an allegorical narrative which illustrates one or more instructive lessons or principles, also known as "parable".
Consequently, the illustration of the parabola in the beginning of the film is there to signify that the title itself is an analogy and to also inform the viewers that we are about to view a parable, a short didactic story.
What do you doubt exactly? Facts? Because when Greeks say "παραβολή", they mean one of two things; parabola or parable. This is the etymology and definition, not an opinion.