The first printing of the AD&D 2nd Edition Player's Handbook (TSR 2101) from 1989 lists the carrying capacities for horses in Table 49 on page 78. "Light" encumbrance causes movement rates to be 2/3 of normal. As an example, this table shows that a Draft Horse would be lightly encumbered moving at 2/3 movement rate while carrying between 261 lbs and 390 lbs.
In the second printing of the Player's Handbook (TSR 2159) from 1995 in Table 49 on Page 105 there is a printing mistake which leaves out the movement rate modification in the column headers. The weight ranges in the table conform to the ranges in the 1989 printing (Draft horse is still 261 to 390 lbs.) so one could assumed that the printing mistake was to leave out the 2/3 and 1/3 movement rate modifiers present in the 1989 printing.
The Monstrous Compendium Volume 1 (TSR 2102) from 1989 says the following on the horses page:
On the following chart, the first number is the animal's normal carry-ing capacity, the second is its encumbered, half-speed load, and the last is its fully encumbered, 1/3 movement load.
The weights in the chart conform to the weights in the two printings of the Player's Handbook but the text indicates that the middle encumbrance level is moderate or 1/2 movement rate, not light or 2/3 movement rate.
The Monstrous Manual (TSR 2140) from 1994 on page 194 in Horse Table 1. agrees with The Monstrous Compendium and contradicts the Player's Handbook, again with the same weight ranges, with a column heading for the center column of Half speed instead of Two-third speed
Finally, just for context, the AD&D 1st Edition Monster Manual (TSR 2009) from 1979 indicates on page 53 that a draft horse can carry up to 4,000 gold pieces in weight or 400 lbs and be unencumbered. It can carry between 4,000 and 8,000 gold pieces in weight or 400 lbs to 800 lbs and be encumbered, moving at one-half speed.
Which books are correct? Does a, for example, draft horse carrying between 261 and 390 lbs move at 2/3 movement rate as indicated in the Player's Handbooks or at 1/2 movement rate as indicated in the Monstrous manuals and compendiums?