Can a Huge bird (STR 15, <1000 lbs) really fly off with a Gargantuan Brontosaurus (>30000 lbs)?

Question

I am a druid shaped into a Quetzalcoatlus, a Huge flying creature. I see a Brontosaurus and know I can grapple a creature up to one size larger (up to gargantuan). To everyone's surprise, I succeed in grappling the mighty beast despite its immense strength.

I decide to try flying off with the Brontosaurus. Can I do it?

This previous answer suggests to me that this is indeed possible, and that carrying restrictions have nothing to do with moving with a grappled creature: Does carrying/dragging a willing/unconscious creature cause half movement as per the grapple rules?

Can I really fly while holding onto a creature weighing more than 30 times my own weight? This could also apply to a large bird flying off with a huge beast, etc.

Answer 1

RAW depends on whether or not your table considers weights of monsters.

The case for only 1/2 movement

Most monsters in the DMG do not have assigned weight values. Because of this, Jeremy Crawford on Twitter has simplified the answer as to moving grappled creatures to size only:

The rule on moving a grappled creature (PH, 195) works regardless of a creature's weight. It cares about creature size. [...] The rule doesn't rely on weight largely because we don't specify weight for most monsters.

The case for applying push/drag/lift rules along with 1/2 movement

But we do know approximately how much these dinosaurs weighed and a DM could utilize that - but they do not have to.

If they do, then what's going on here is the interaction of two separate rules: those for grappling and those for pushing/dragging/lifting. We need to look at each and how they work together to answer this.

The rules on pushing, lifting, and dragging state:

Push, Drag or Lift. You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet.

Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift

The Quetzalcoatlus has a strength of 15 (as noted) and is also Huge, which means it's maximum drag/lift weight is 1800lbs. Because the weight of the Brontosaurus exceeds that, your movement is dropped to 5'.

Dragging/Lifting/Carrying a Grappled creature

This answer covers this, but to summarize, the language for grappling states (my emphasis):

When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

This specifically states the terms drag and carry which are the mechanical terms described above and should therefore still apply. Your movement is still halved, so you'd only be able to move 2.5 feet.

Answer 2

RAW: Yes you can

Jeremy Crawford (lead rules designer) has tweeted this unofficial ruling:

Q: is pushing/dragging a grappled creature subject to the carrying capacity rules?

A: The rule on moving a grappled creature (PH, 195) works regardless of a creature's weight. It cares about creature size.

The rule on grappling says on the subject:

Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you. (PHB 195)

As opposed to the rules for Lifting and Carrying (PHB 176) which applies to objects.

Now as a DM I would apply some common sense restrictions on this especially if you want to take the Bronto for a flight. I would probably call that you can drag it 5 feet (as the rule of pushing/draging something past your carring capacity)

Answer 3

Drag OR Carry

Do weight rules apply?
I am on-record as saying that the Grappling Rules are for creatures, and the Lifting and Carrying rules are for objects. I don't think the weight of the graplee should be considered when adjudicating a grapple, just its size. However, I would point out that the specific game terms drag and carry are not explained in the Grappling rules, but rather in the Lifting and Carrying Rules. There, we find that one can drag or lift weights that are twice what one can carry. This is enough to make me question whether it is as easy for a grappler to carry an opponent as it is for them to drag the same opponent - or in this case, whether there are opponents that they can drag, but not carry, albeit based on size, not weight.

Personal RW Experience
As a DM, I find that the players at my table appreciate it when I interpret rules in such a way that supports verisimilitude in the game. The martial art I practice involves a lot of grappling. I routinely drag creatures around which are larger than I am. On occasion, I even lift them, typically for a hip throw. However, I don't carry creatures that are larger than I am.

The grappling rules state:

Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

Let's focus on the 'or' in 'drag or carry'.

The grammar of CAN / OR
The combination of 'can' with 'or' may indicate a completely free choice, but one in which you are not permitted both:
When you reach the T-intersection, you can turn left or right
(but you can't simultaneously turn left and right).

However, the combination of 'can' with 'or' may also indicate a situation in which you will not necessarily have a free choice, but the possible outcomes are unknown because the initial conditions have not yet been determined:
Mrs. White is allergic to shellfish and might be attending dinner.
Miss Scarlett is allergic to peanuts and might be attending dinner, but will not attend dinner if Mrs. White does.
We have only one sauce dish.
Depending on who attends, we will serve either the shrimp sauce, or the peanut sauce, but not both.

When the guests arrive, we can serve the shrimp sauce or the peanut sauce.
Here we do not have a free choice, but unknown future conditions which force us to use 'or'.

We can serve the shrimp sauce or the peanut sauce.

You can drag or carry the grappled creature with you.

Do I think the grappling rules actually intended the second grammatical interpretation? I honestly have no idea, and certainly can't prove that they did. However, taking them as if they did is one valid interpretation of RAW, dovetails nicely with the use of drag and carry given in the rules, and adds verisimilitude. I believe ruling this way would improve the playing experience.

Moving a Grappled Creature.
When you move, you can drag or carry the grappled creature with you, subject to its size:
When grappling a creature that is a size larger than you, you may only drag it.
When grappling a creature that is your size, you may drag or carry it (your choice).
When grappling a creature that is one size smaller, you may drag or carry it (your choice).
When grappling a creature that is 2+ sizes smaller than you, you may only carry it.

Flapping its wings mightily, your Quetzalcoatlus may drag the brontosaurus across the field, but it may not take it into the air and carry it away.