CreateEllipticRgn, CreateRectRgn, CreatePolygonRgn, CreateRoundRectRgn, CombineRgn. SetWindowRgn

TMuBytton = class(TButton)
 protected
   procedure CreateWnd; override;
end;

implementation

procedure TMuBytton.CreateWnd;
var p: array[0..2] of TPoint;
    R:HRGN;
begin
 inherited CreateWnd;
 p[0] := Point(0,0);
 P[1] := Point(width,Height div 2);
 p[2] := Point(0,Height);
 R := CreatePolygonRgn(p,sizeof (p) div 8,WINDING);
 SetWindowRgn(handle,R,true);
end;