The one-third probably of what An Sheng said was correct. However, that was the global drop rate across different Mythical creatures. They had never killed the same Mythical creatures consecutively.
In reality, killing a Mythical creature might result in a Mythical creature of the same type taking a long time to come through the dimensional rift. It could take a year or decades. There wasn't much chance of killing the same Mythical creature.
Some families were rather lucky. They spent very little time researching a Mythical creature. After killing it, they would have Mythical creatures of the same type appear every year for years.
Even after killing many Mythical creatures, the chances of a Companion Egg dropping remained very low. Only when one was extremely lucky would a second Companion Egg of the same type drop.