The actual quote is "A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load."
This can be read in one of two ways:
1) It can only move through the air if carrying no more than a light load: the reference to "the indicated speed" is just because this rule is talking about creatures with a listed fly speed... It's badly written, is all.
2) It can still fly provided it can maintain minimum forward speed in a round, but its movement rate is reduced if carrying more than a light load... It's badly written, is all.
The evidence in favour of the first reading is:
a) The FAQ says so
b) The Rules of the Game articles say so
c) The
Reinforced Wings feat description:
Races of the Dragon said:
Benefit: You can fly in medium armor or with a medium load. Your speed is still reduced by encumbrance due to armor or total weight.
Normal: Flying creatures can’t fly in medium or heavy
armor or when carrying a medium or heavy load.
e) The final and probably the most compelling argument, preceded by a brief history... The original 3E Monster Manual had the same sentence as the one I quoted initially but with "medium load" instead of "light load", which was confirmed by the Sage (yeah, I know, but bear with me) to mean that a creature couldn't fly with greater than a medium load (rather than meaning that its speed would be reduced if it did so). In Monster Manual III, this was corrected to "light load", and Masters of the Wild further clarified the position. The reconstructed passage in Masters of the Wild was eventually pasted
exactly into D&D 3.5, theoretically ending the argument...
The trouble is
where it was put in D&D 3.5: you'll find it in the DMG, page 204, under the section on Unusual Mounts!
DMG said:
Suitable Mounts: You have the final decision on what is or is not a suitable mount. At its most basic level, a mount should have the following characteristics:
• Able and willing to carry its rider in a typical fashion. (A camel is able and willing. A tiger might be capable but may not be willing. A giant might be willing but not truly able.)
• At least one size category larger than the character. Also, a flying mount can carry no more than a light load aloft.
• The mount’s Challenge Rating should be no more than 3 less than the rider’s character level. If the mount can fly, its Challenge Rating should be no more than 4 less than the rider’s character level.
Doubtless this is where StreamOfTheSky remembers the information about flying mounts from. The emphasis on the carrying capacity of flying mounts is underscored by the accompanying table footnote:
DMG said:
Carry: The weight of the heaviest load the mount can carry—the light load limit for a flying mount, or the maximum load (heavy load limit) for a nonflyer.
(Incidentally, the table
also definitively answers the question of whether a flying quadruped's weight capcity is increased in the same way as a ground-bound one: whether or not you think it makes sense, the answer is unequivocally "yes".)
So, the question as to how much a flying
mount can carry whilst flying is answered. However, if you want to be really pedantic about it, this passage strictly doesn't cover flying creatures that aren't acting as mounts. Nonetheless, I think the inescapable conclusion is that it was intended that creatures should not be able to fly with more than a light load.