In the Monster Manual, Winged Kobolds have the Dropped Rock attack, which does damage to one creature directly below it. I know the Kobold doesnt usually have a bag of holding full of large rocks (even though I could make that happen). Can I, as DM, reasonably interpret that to allow the Kobold to pick up any similarly massive object to drop on an enemy?
1\$\begingroup\$ There are rules about how much a person or monster can carry. There is a good answer about that: Can a monster with a flying speed lift a Grappled PC and then drop them? \$\endgroup\$– Alexis WilkeFeb 8, 2015 at 13:26
Before I get into the meat of the answer, it needs to be stated: You're the DM, you can do whatever you want. Especially when it comes to reskinning without changing anything mechanically. (Even players can do that.)
Dropped Rock. Ranged Weapon Attack: +5 to hit, one target directly below the kobold. Hit: 6 (1d6 + 3) bludgeoning damage.
Notice that there's no limit on ammo mentioned, suggesting that the intent is not for the kobold to be carrying the large rock(s) around. While we don't have a clear statement here, it seems unlikely that the kobold magically produces large rocks at will. Therefore, it can be presumed that it's picking them up from its immediate surroundings. That being the case, it seems reasonable that one large heavy object is much like another, and the kobold can use whatever comes to hand.
I assume the attack says "rock" primarily because there will generally be rocks in the vicinity of most places you'd expect to see a kobold, and "Dropped Large Heavy Object" just doesn't roll off the tongue.
It should be noted that, while you as the DM are free to rule however you please, the "Dropped Large Heavy Object" attack should probably have the same stats as the "Dropped Rock" attack regardless of which object you actually use. It's already a fairly powerful attack for a CR 1/4 creature, so giving it a damage bonus because the kobold picked up a spiky rock (for example), might not be the best idea.
3\$\begingroup\$ On the other hand, if a PC manages to somehow enslave a winged kobold and has a specially-designed spiked rock made by a local smith - Well, feel free to give them points for effort. \$\endgroup\$– GMJoeFeb 9, 2015 at 5:31
\$\begingroup\$ You could apply the lift and carrying capacity rules to see how much the Kobold could carry. Make an educated guess about the difference (as a multiple) in weights of both rocks and apply that as a modifier to the damage if you really wanted. However, be mindful of adjusting the Kobold's CR if you do. \$\endgroup\$– EricFeb 9, 2015 at 8:42