Tailgate is a minibot recently (accidentally) uncovered from the Mitteous Plateau; seems he was stuck down there for a few million years. To make things worse, he totally missed the launch of the Ark-1. Now he's just trying to figure out this new world he has landed in, and his place in it.
Tailgate's records begin three million years ago; born in Rivet's Field on Cybertron two weeks before the launch of the Ark-1, Tailgate was assigned to a low ranking sanitation role and given the job of cleaning the Ark-1's coolant tubes before take-off. He was late to the job-- and in the end, Tailgate never made it there at all. If anyone noticed, they never looked for him. As a low caste bot with a basic role, Tailgate was unimportant enough to be subsequently forgotten in the ongoing flow of Cybertronian time.
What's a little bot to do?
Yarn-spinning, waste disposal procedures. Maybe bombs too. Ingenuousness. Making it up as he goes. That works, right?