Because I am old , I remember how things "used to be"
Back in the dark ages grub screws had square heads on them ( 4 sides ).
The railways used rectangular ones ( early tamper proof head ) and some busses had penta ( 5 sided) ones on the engine pulleys to prevent theft .
CCTV has rendered these extremes redundant now days but you might actually have one of these fitted.
Around the 70's to 80's they got replaced with hex holes.
Good chance you have an old square one that some one tried to remove with the wrong tool.
If worst comes to worst you can get extractors for removing rounded off Posi Drive & Philips headed screws.
There are good ones , made from tool steel and there are toy ones made from modified plasticine in China and sold at Harbour Freight, Wally mart, Lowes etc.
Go to a specialist machine tool shop or bearing shop and buy a good set.
Some places sell then as single heads for impact drivers and these are the best.