Echo 136 was the serial number of a Pelican dropship attatched to the UNSC Pillar of Autumn in 2552. When that vessel crashed on Installation 04 that year, the dropship's pilot, Lieutenant "Cookie" Peterson, launched it as the Autumn was going down, effectively saving it from destruction.
The ship then met up with other survivors at Alpha Base where it was assigned to ferry troops and supplies from the Autumn crash site. Its sister ship, Echo 206, had the same assignment.
Later, when the ODSTs chose to assault the Truth and Reconciliation, "Cookie" was assigned to pilot Echo 136 into the bays of the ship and unload troops.
NOTE: The events surrounding the destruction of Echo 136 were never described, so there are two possibilites to how it was destroyed; either it blew up when the Truth and Reconciliation was destroyed, or it was the sole dropship to survive Installation 04, and was thus destroyed during the Master Chief's attack on the Ascendant Justice. It is more likely that the Pelican the Master Chief used was Echo 206 since "Cookie" was not at the controls of that ship, but it is still possible, as -136 and -206 were the only two dropships not mentioned as destroyed during Halo: The Flood.