The dragonspawn here are elites and in the level range of 51-53, so it is certainly the most difficult of the Whelpling pets to farm. People who are used to farming in Outland will have no trouble with them however.
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.
Probability of at least one drop in X kills
Number of Kills
Neither the drop rate nor cumulative probability increases based on the number of monsters you have already killed.