Animosity - We’ll show ‘em

Front arc does not necessarily imply LOS and vice versa. They are two distinct and independent requirements. As BRB p. 10 specifies: "Line of sight determines what a model can 'see'" (ergo, what is visible). "For one model to have line of sight to another, you must be able to trace an unblocked line from its eyes to any part of the body (i.e. the head, torso, arms or legs) of the target (ibidem).

If you can draw LoS to a unit outside the forward arc, it is still visible. However, you cannot e.g. charge or shoot at it, because these rules specifically require the target to be in the forward arc. You definitely cannot draw LoS to a unit behind you.

Ahem...


Unless specifically stated otherwise, normal rules apply. The rules for unit facings (BRB p. 5) are not about what the model can see, but "about what the model can attack, as well as calculating which side of the model an enemy will be able to charge as the game goes on." The rules for LoS are about "what the model can see." [Italics mine]

The rules for magic missiles do not support, but actually refute your position. All spells (unless specifically stated otherwise) require the target to be in the forward arc to attack it - but they do not need to see it. However, "magic missiles do require the Wizard to be able to see his target. Accordingly, a Wizard can only cast a magic missile at a target that lies within his forward arc and to which he can trace line of sight (just as if the wizard was firing a missile weapon)."

Likewise (but not quite so obvious), the rules for shooting specify that being in the forward arc is not enough for that unit to be attacked, the attacker must be able to draw LoS to it. 

Both are required for the purposes of shooting.

Once again: unless specifically stated otherwise, normal rules apply. The general rules (BRB p. 10) tell us that "Line of sight determines what a model can 'see'. [...]Line of sight literally represents your warriors' view of the enemy — they must be able to see their foe through, under or over the battlefield terrain, and other models (friendly or enemy). For one model to have line of sight to another, you must be able to trace an unblocked line from its eyes to any part of the body (i.e. the head, torso, arms or legs) of the target. Sometimes, all that will be visible of a model is a weapon, banner or other ornament he is carrying. In these cases, the model is not visible."  

There is no requirement for forward arc, and the general rules for facings do not contain such a requirement either. In fact, the latter do not mention "seeing" (or somesuch) at all.  

The rules for shooting specifically state otherwise. However, there is no shooting in the We'll show 'em special rule, and the enemy unit in the first part of the We'll show 'em special rule is not a target, although it can become one later, if and when a charge can be declared. Indeed, those rules imply that visibility does not require forward arc: "Pivot the unit on the spot to face the nearest visible unit" - precisely to bring it in the forward arc. 

Don't all fast cavalry middle troops?

Supposed to be "missile" troops. I'm on my phone and got auto corrected.