underdog (n.)

"the beaten dog in a fight," 1887, from under + dog (n.). Compare top dog "dominant person in a situation or hierarchy" (see top (adj.)). Its opposite, overdog, is attested by 1908.

I'm a poor underdog

But tonight I will bark

With the great Overdog

That romps through the dark.

[from "Canis Major," Robert Frost, 1928]