First of all make sure that this scrip http://www.teatar.hr/js/idangerous.swiper-2.1.min.js linked before your header swiper initialization code.
Second, if you initialize Swiper in header, you should do it within $(document).ready like:
<script>
$(document).ready(function(){
var mySwiper = new Swiper('.swiper-container',{
pagination: '.pagination',
loop:true,
grabCursor: true,
paginationClickable: true
})
jQuery('.arrow-left').on('click', function(e){
e.preventDefault()
mySwiper.swipePrev()
})
jQuery('.arrow-right').on('click', function(e){
e.preventDefault()
mySwiper.swipeNext()
})
})
</script>