There May Be Some Who've Wondered Why the Rat

There May Be Some Who've Wondered Why the Rat

There may be some who've wondered why the rat
Happens to be first to have a year
Exclusively devoted to his name.
Yet clever, crafty creatures never fear:
Eventually they'll win -- that's where they're at!
And this is how it came about: The rat,
Racing for the prize, fell towards the rear.
Out of breath, he thought he'd lost the game.
Fast rampaging Bull was drawing near;
The problem was: How to reach his back?
Here came by the lost, high-leaping cat.
Eureka! Up they went, the two friends dear,
Riding on the bull's back towards fame.
At the last, Rat pushes Pussy clear,
Then leaps ahead of Bull -- and that was that!

Question: why is the rat considered the first?