✓ Mathematica : cpu = 0.290986 (sec), leaf count = 26
DSolve[-x^2 - 2*x*y[x]*Derivative[1][y][x] + (-a^2 + x^2)*Derivative[1][y][x]^2 == 0,y[x],x]
✓ Maple : cpu = 0.737 (sec), leaf count = 51
dsolve((-a^2+x^2)*diff(y(x),x)^2-2*x*y(x)*diff(y(x),x)-x^2 = 0,y(x))