Again, don't use a random value.
You should specify to test your origin as reference + some units forward. It will spawn a zombie where you aim 150 units forward you.
Code:
new Float:Angles [ 3 ];
new Float:Origin [ 3 ];
new Float:Forward[ 3 ];
pev( Player, pev_origin, Origin );
pev( Player, pev_v_angle, Angles );
angle_vector( Angles, ANGLEVECTOR_FORWARD, Forward );
xs_vec_mul_scalar( Forward, 150.0, Angles );
xs_vec_add( Origin, Forward, Origin );
new Zombie = create_entity( "monster_zombie" );
set_pev( Zombie, pev_origin, Origin );
DispatchSpawn( Zombie );