Face the Way You Move

transform.LookAt needs a point to look at. Often what you have is a direction instead: a velocity, the wind, the way something moved. Quaternion.LookRotation(direction) returns the rotation whose forward (the blue z arrow) points along that direction, with the object's up kept toward the world's up. An arrow shot into the air points up at first and tips over as it falls, because its velocity does:

Rigidbody rb;

void Start()
{
    rb = GetComponent<Rigidbody>();
}

void Update()
{
    if (rb.linearVelocity != Vector3.zero)
    {
        transform.rotation = Quaternion.LookRotation(rb.linearVelocity);
    }
}

Only the direction counts, not the length: (1, 0, 2) and (3, 0, 6) give the same rotation. A zero vector has no direction, so LookRotation(Vector3.zero) logs Look rotation viewing vector is zero and returns Quaternion.identity, which snaps the object to face +z. That is why the arrow checks first: once it stops, it keeps the facing it had. LookAt(target.position) gives the same rotation as LookRotation(target.position - transform.position).

The direction's up and down part tips the forward up or down with it. For something that must stay upright, such as a walker going up a slope, set the direction's y to 0 first, so the object turns only around y. A second argument, LookRotation(direction, Vector3.up), says which way is up; Vector3.up is what you get without it.