There are a couple marksmen (ADCs) that can be brought mid.
Along with the ones Jelly Mac listed (Ezreal, Varus, Urgot, and Corki), Jhin and Quinn can be brought mid.
Ezreal is a slippery, long ranged caster who can either build attack damage or ability power when going mid, and is therefore extremely versatile. He lacks waveclear, however.
Varus is a long ranged sniper with no escapes but terrifying long ranged damage. Since he's being played mid, he doesn't rely on his auto attacks as much as he does on his spells.
Urgot is a medium ranged anti-assassin middle laner. He does lots of sustained damage with his auto attacks and the Acid Hunters that he weaves in between them.
Corki is a hybrid damage medium ranged marksman who relies almost as much on his abilities as he does on his auto attacks.
Jhin is another long ranged sniper. He like Varus except he relies on his auto attacks more because his poke doesn't do as much damage and is longer cooldown.
Unlike these other marksmen which build like AD casters (relying mostly on spells to do damage), Quinn builds more like an assassin when played middle lane, jumping on people and doing bursts of damage with her spells and auto attacks.
Aside from Ezreal's AP build and Corki's hybrid build, all of these champions build attack damage. Attack damage champions can be brought mid, but they're rarer than ability power champions. This is partly due to the fact that ability power champions fill the niche that middle lane requires better, and partly due to the fact that the other lanes find it harder to choose sources of AP. Most top laners are attack damage based, aside from a few ability power based tanks such as Maokai, Galio, Nautilus, and Cho'Gath, some close ranged mages such as Karma, Lissandra, Lulu, and Ryze, and a couple of other hard-to-classify champions like Rumble, Teemo, and Singed. Jungle is mostly attack damage fighters, though there are also ability power assassins like Elise, Fizz, Ekko, and Diana, ability power tanks like Nautilus, Cho'Gath, and Sejuani, and Gragas, and then odd champions like Fiddlesticks. Most supports deal ability damage, though only few itemize for more, such as Karma, Annie, and Brand. Most bottom laners don't do ability damage; the only two that do noticeable ability damage are Corki and Mordekaiser.