Right. These are pretty close to being bursts (I mean, AD&D cones could have varying ratios of width to length, compared to the uniformity of 5e).In Moldvay Basic (B/X) a Green Dragon's and one of the Gold Dragon's breath weapons was gas in a cloud shape, but it had to be directly in front of the dragon, not a ranged burst.
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In AD&D clouds are similar, green dragons and metallic dragons have cloud breath weapons with rectangular dimensions.
1e MM page 33:
A green dragon can attack by a claw/claw/bite routine or by breathing a cloud of poisonous chlorine gas. The gas cloud is 5” long, 4” wide, and 3” high.
The classic D&D cloud is reasonably close to a 20' R burst at a range of 25' from the dragon.