Roderick is an elderly hippopotamus. He has practical and symbolic importance for Ankh-Morpork, in that the hippo is the creature metaphorically associated with the city in much the same way that ravens are historically and mythologically linked to the Tower of London. It is said in Ankhian folklore that if danger ever threatens the city, the ornamental wooden hippos on the Brass Bridge will get up and run away. Perhaps this is a garbled take on a variation of the sort of myth that says the Tower of London will collapse if its resident ravens take it into their heads to fly away and leave. (Every city over a thousand years old must have a variation on a theme of this myth - that if the patron animal ups and leaves, great evil will befall. Berlin, for instance, has a similar thing about bears: interestingly, the last bear in the Berlin city zoo died in an air raid in 1944. Less than a year later, the Russians were in town playing catch-up for Operation Barbarossa.).
The association between a long-established city and its patron animal is a strong thing, mandated by narrative causality, and is therefore not to be scoffed at. Ankh-Morpork's animal insurance policy is a pair of hippopotami, normally kept at the College of Heralds on Mollymog Street. However, following a mysterious fire that took place during the arrest of the Dragon King of Arms, the menagerie of animals normally resident at the College was taken into the concerned care of Lady Sybil Ramkin, and Roderick and his friend Keith were last heard of, having happily taken possession of the ornamental lake in the Ramkin House gardens. Knowing Samuel Vimes has a little hobby, concerning making access to the Ramkin house, for any visiting Assassins, as dangerous, difficult and potentially undignified as is humanly possible, then what's the betting that a creature weighing several tons, which is capable of biting a man in two when annoyed, may still be there?
While London carefully places a finger on the scales of Fate by clipping the wings of its ravens, the Ankh-Morpork equivalent of a similar insurance policy perhaps consists of the fact the Roderick and Keith are perfectly happy where they are, and are in any case quite sedate elderly hippopotami.
Roderick and Keith also have a practical as well as a symbolic importance. If they died or disappeared, nobody at the College of Heralds would know how to draw one, which is a drawback, as the city's coat of arms features a shield help aloft by two hippopotami rampant. Suggesting to the literally-minded Heralds that they copy one out of a book, or just copy an existing design, would be met with blank uncomprehending looks... Roderick and Keith are therefore trained, on the command Ooop-La!, to stand upon their hind legs, holding a shield between them, for as long as it takes for someone to do a drawing.
The exact symbolic importance of hippos to the area seems unusual, as they would presumably be found in Howondaland. Possibly a humorous take on the use of lions as symbols, although a bloated but deceptively mean and vicious creature which spends its time fouling up a river is perhaps not an alien concept to the population.
It should be noted that hippo bones have been found in the South of England and dated to an interlude between Ice Ages. Either changing climate or human intervention killed our native population off a mere 6-7,000 years ago...