He did not recognise it as a rat, because of the size.

His horse gave a bound as the thing dropped into the road beside it.

The little lane woke into tumult at the report of the whip and the doctor's shout.

The whole thing suddenly went fast.
_Rattle-clatter, clash, clatter_.
The doctor, one gathers, stood up, shouted to his horse, and slashed with all his strength.

The rat winced and swerved most reassuringly at his blow--in the glare of his lamp he could see the fur furrow under the lash--and he slashed again and again, heedless and unaware of the second pursuer that gained upon his off side.
He let the reins go, and glanced back to discover the third rat in pursuit behind....
His horse bounded forward.