A midget was presumably captured by bounty hunter Jubal Early. According to Early, he was "deadly" and "loved fire," as he performed arson at least once. He was also considered by Early to be the smallest cargo he had ever had to transport right before River. Early tells Simon Tam about the midget while in Serenity's cockpit.
Chiasmus Reference Edit
The plot of the episode "Objects in Space" contains many elements which link the Early half of the episode to the River half of the episode. In the commentary for the episode Joss Whedon stops at the line mentioning the this individual as "one of my favorite exchange that I’ve ever written". Since most of the episode fits this form, the reference can be a twisted parallel earlier conversation:
Early: "With the exception of one deadly and unpredictable midget, this girl is the smallest cargo I've ever had to transport, yet by far the most troublesome. Does that seem right to you?"
Simon: "What did he do?
Simon: "The midget."
Early: "Arson. Little man loved fire."
This discussion is had while Kaylee is unbolting the crew quarters from the outside. The earlier conversation starts with Mal suggesting they should start doing that to River.
Zoe: Where's River at now?
Mal: In her room. Which I'm thinking we bolt from the outside from now on.
Wash: That's a little extreme isn't it?
Jayne: Anybody remember her coming at me with a butcher's knife?
Wash: Wacky Fun.
Jayne: You want to go little man.
Wash: Only if it's someplace with candlelight.
Zoe: Sir: I know she's unpredictable. But, I don't think she'd harm anyone.