The revised 3rd edition (3.5) removed the longspear from the table of
available weapons.  It still exists in Fivtoria.

----
The ballista as written up on DMG page 151 is nothing like the 1st
Edition version, which did so much to shape the world.  Below is an
attempt to port the 1st Ed version, along with other weapons in the
same group.  The simple/martial/exotic distinction is due to the
mechanics of making a bow that big, and are unrelated to the shooter's
size.  When using any crossbow more than one size larger (i.e. a human
with a ballista), the weapon is too unwieldy to aim dynamically, so
the operator's base attack bonus or similar attack bonuses are not
considered in the attack roll.  (The Dexterity bonus does count,
though.)  When using an unmounted crossbow exactly one size larger
than oneself, use the modifiers for firing a heavy crossbow in one
hand, from PHB page 100.  A crossbow two sizes smaller than oneself
can be used as a "hand crossbow."

Large crossbow (arbalest), simple weapon
  cost: 200 gp
  damage: d12(+5)
  critical: x3
  range increment: 130 ft.
  special: bypasses up to +5 armor bonus to AC (less 1 per range increment),
	manned and mounted assume 20 Strength,
	loading/cocking takes 1 person 2 full-rounds (and if
	someone else loads, it takes a move-equiv to take over)

Huge crossbow (ballista), martial weapon
  cost: 500 gp
  damage: 2d6(+10)
  critical: x3
  range increment: 140 ft.
  special: bypasses up to +10 armor bonus to AC (less 2 per range increment),
	manned and mounted assume 30 Strength,
	loading/cocking takes 1 person 3 rounds or 2 people 2 rounds,
	or 3 people 1 round and 1 move-equivalent action.

Gargantuan crossbow, exotic weapon
  cost: 5,000 gp
  damage: 2d8(+15)
  critical: x3
  range increment: 150 ft.
  special: bypasses up to +15 armor bonus to AC (less 3 per range increment),
	manned and mounted assume 40 Strength,
	takes much more skill and time to operate.

The ballista mechanic of 1st Edition was effectively a ranged touch
attack, inconsistent with how smaller crossbows were handled.  The
house rules above result in the same inconsistency.  They suggest that
Strength bonus should apply to all ranged attacks, but that makes it
too much easier to hit with a missile weapon than with a melee weapon.
Perhaps the distinction is that these large crossbows have big, heavy
missiles that can convert their initial momentum into penetrating
power better than smaller missiles can...