Gosh, I thought it looks kind of normal...or at least, not too unusual. I can't quite make it out though so, maybe I am missing something...I can see that it is not a perfect rectangle like other horses have.
( I think it's funny, in the first picture, I can see your reflection in his eye! )
If you're really concerned, then 'yes', I would have a vet look at it. However, I personally, don't think it looks that weird.