The Tiny Crimson Whelpling drops from any mob within the Wetlands at an extremely low rate.
Tiny Crimson Whelpling dropped from Crimson Whelps, Flamesnorting Whelps, and Red Whelps in the Wetlands. There were also Lost Whelps in the general area, but they did not drop this pet. The level of these whelps ranged in the mid-20s, so this was the first whelpling pet new characters had the chance of obtaining, and also the easiest for higher level characters to farm.
The drop rate of this pet is 0.1% (one in every thousand kills) and takes an incredible amount of patience and determination in order to get one. Due to the difficulty of obtaining one, and thus their rarity, they can sell for fairly large amounts of gold on the Auction House, at least as far as pets are concerned. The table below shows the cumulative probability of getting at least one after a number of kills.