|Alighieri basically categorized sin into three severities: Incontinence, Violence/Bestiality (aka "Sins of the Lion"), and Fraud/Malice (aka "Sins of the Leopard"). The first four rings are reserved for the Incontinent, while the other two categories are housed in the "City of Dis".
The River Styx serves as an actual barrier between Upper Hell (Rings One through Four) and this City of Dis / Lower Hell (Rings Five through Nine).
The Fifth Ring houses "the Wrathful", those who are consumed by their anger. These angry souls are essentially stuck in the River Styx and cursed to ruthlessly tear each other apart for eternity.
I chose to use the senseless conflict between cats and dogs to illustrate the wrathful, and considering how the animals actually react when immersed in water, I thought the pairing was perfect