Were those bizarrely shaped arrowheads used by the Persians in the movie 300 real, or were they just the product of a prop master's overactive imagination?
Like much of what appears in the movie, they're the product of a prop master's overactive imagination.
The arrowheads in general use at the time were of two main types: socketed bronze heads, usually tri-lobate (like a modern three-bladed broadhead), and iron tanged flat (i.e., bi-lobate) heads. Some iron heads have a prominent central ridge, and some appear diamond-section. It's hard to tell with many of the iron heads, since they're usually much more corroded than the bronze heads.
The arrowheads actually found at Thermopylae unsurprising conform to this general pattern.
For some nice photos of arrowheads from Thermopylae taken at the National Archeological Museum in Athens in 2014, see:
Note that the larger heads are javelin heads, not arrowheads.
A clearer photo of iron arrowheads: https://www.livius.org/pictures/greece/thermopylae/thermopylae-arrowheads/