About

Tom is the name Rarity gives a large rock in The Return of Harmony Part 2. Tom Dan is a meme that started from bronies calling the boulder Dan before the episode aired. Once it aired and Rarity called it Tom, the bronies dubbed it Tom Dan.
“Look out! Here comes Tom” is a YouTube video meme.

Origin

The Return of Harmony is a two part episode of My Little Pony Friendship is Magic. Rarity, while under the influence of Discord in The Return of Harmony Part 1, digs out a large diamond from a rock wall with three gems reminiscent of her cutie mark embedded on it. Only Rarity sees it as a diamond. When Twilight arrives, the "diamond" is only a simple boulder. Rarity refuses to part with it for the majority of both episodes.
At Twilight's library, Rarity remains outside, and says she's on to Twilight's plan to steal "Tom" from her, addressing the boulder by name.

Tom Dan

Prior to part two, the boulder was dubbed "Dan" by fans due to Youtube captions. When episode 2 aired and Rarity named it Tom, fans decided his name was Tom Dan.

Tom Dan Does All His Own Stunts

Someone had added glasses to Tom Dan and an action star was born.

Exploitables

Video Example

Look out! Here comes Tom

"Look out! Here comes Tom" is a YouTube video parody. The first uploads were on September 24th,2011. YouTube has several variations of it with different Toms. It's a scene from "The Return of Harmony Part 2" where Twilight magically levitates the rock and is heard exclaiming "Look out! Here comes Tom!" before the boulder flies out the window and falls to the ground. In the YouTube parodies the boulder is replaced with the face of a well known person named Tom.

Parodies