I was at a kaiten-sushi the other day and a friend visiting from NY asked what the "kappa" was in "kappa-maki" so I told her that the river imp in Japanese folklore, the kappa, likes to eat cucumber, which is the main ingredient in a kappa-maki.
However, I couldn't answer her question about the tekka-maki. I looked up tekka and the dictionary only had "red hot iron" so the etymology wasn't clear to me. I looked it up on wikipedia and here is the entry:
"Tekkamaki (鉄火巻き) is a kind of Hosomaki filled with raw tuna. Although some believe that the name "Tekka", meaning 'red hot iron', alludes to the color of the tuna flesh, it actually originated as a quick snack to eat in gambling dens called "Tekkaba (鉄火場)", much like the sandwich."