Monster weapons rules differ from Player weapon rules.
A medium or small creature using a javelin-like weapon does 1d6 +4 damage if the creature's Strength is 18 (Merrows have 18 Strength). The Merrow, being a large creature, adds one additional damage die to its Merrow-sized weapon to arrive at 2d6 + 4. This makes it a unique weapon for this creature.
Why the added die?
In the DMG (p. 278) damage section of Building a Custom Monster the larger sized weapons double / triple / quadruple the damage a monster's weapon does if the monster is large, huge, or gargantuan. (From the base damage being done by a medium sized creature (thanks to @Carcer for the page citation. See creature size chart at the end). All PC classes are medium (or small) sized humanoid creatures in the PHB.
You can see this in the great sword that a fire giant uses - it is a (huge) fire giant-sized weapon not normally usable by medium-sized humanoids.
Fire Giant {snip} Multiattack. The giant makes two greatsword attacks. Greatsword. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: (6d6 + 7) slashing damage. (MM, Fire Giant)
Yikes, that's a lot of damage! With a strength of 25 (+7 mod) and a size of huge, the damage was calculated like this: huge is two sizes larger than medium so you triple the basic 2d6 great sword damage (PHB). A large creature with a large-sized great sword would have a 4d6 (+ Strength Mod) as the damage base.
The merrow is described as a
Large monstrosity
in the MM so its damage was derived in the same way: 1d6(javelin/harpoon) + (strength Mod), with weapon die doubled, making it 2d6 + 4.
Why not a harpoon/javelin with a rope?
As to the "harpoon/javelin with a rope attached" - this isn't a standard weapon, so you'd need to work with your DM to tailor attaching a rope to harpoon/javelin. If the "hook" catches, offer an opposed Strength check to resist being pulled toward the PC. A key difference is in the hooked end of the harpoon that makes for the "stay attached to the target" feature. The PHB lists standard weapons, so this variation on a javelin needs to be worked out at your table.
Hey, we just defeated a Merrow, I want to use this harpoon!
You've got a specialized javelin that you need to train with, which is a perfect opening for a downtime activity (Training) as described in the PHB (p. 187) and in the DMG (p. 231). Work out the time, expense and other details with your DM.
Or
The PC can use it and attack with disadvantage.
The same part of the DMG that discusses making monster weapons(p. 278) says that creatures can use a weapon for one size larger than they are, but the attack is with disadvantage. This is similar to, and consistent with, a halfling or gnome (small) PC using a great sword with disadvantage due to its "heavy" property in the PHB.
Heavy. Small creatures have disadvantage on attack rolls with heavy
weapons. A heavy weapon's size anf bulk, make it too large for a
Small creature to use effectively. (PHB, p.147)
A good ruling would be "use it, attack with disadvantage" ... RAF is complied with, and RAW in the DMG supports it.
Size (MM)
A monster can be Tiny, Small, Medium, Large, Huge, or Gargantuan.
\$\begin{array}{|l|c|l|}
\hline
\textbf{Size} & \textbf{Space} & \textbf{Examples} \\
\hline
\text{Tiny} & \text{2½ by 2½ ft.} & \text{Imp, Sprite} \\
\text{Small} & \text{5 by 5 ft.} & \text{Giant Rat, Goblin} \\
\text{Medium} & \text{5 by 5 ft.} & \text{Orc, Werewolf} \\
\text{Large} & \text{10 by 10 ft.} & \text{Hippogriff, Ogre} \\
\text{Huge} & \text{15 by 15 ft.} & \text{Fire Giant, Treant} \\
\text{Gargantuan} & \text{20 by 20 ft. or larger} & \text{Kraken, Purple Worm} \\
\hline
\end{array}
\$
