The child satyr was conjured up by Lucien (as was a wind that downed Kirk) in response to Kirk asking why he and his fellow officers from the USS Enterprise had been brought to the planet. The boy had a spinning top that stopped rotating as Kirk looked at the child and Lucien then commented, "Children will play." (TAS: "The Magicks of Megas-Tu")
In the script of "The Magicks of Megas-Tu", this being is twice referred to, both times as a "little boy satyr." The script includes some actions that were written for the child satyr but which are not shown in the episode's final version. Specifically, the teleplay states, "[He] smiles up at Lucien, waves, then picks up his toy, hurries off."
The Japanese name for the episode "The Magicks of Megas-Tu" references this type of lifeform; the title, translated back into English, is "The Satyr on the Magic Star Megas-Tu". (Star Trek Concordance, 2nd ed., p. 84)