what do the men bet on?

The men would let animals loose (rats, cats, or chickens) and bet on what the Hound would catch first.

"At night when things got dull, which was every night, the men slid down the brass poles, and set the ticking combinations of the olfactory system of the Hound and let loose rats in the firehouse area-way, and sometimes chickens, and sometimes cats that would have to be drowned anyway, and there would be betting to see which the Hound would seize first."