There have been a certain number of animals in advertising. Here is a list of which animal goes to which company. Mostly there is no rhyme or reason to it.
Bear: Used to advertise beer-Hoffmeister in the 1980s.
Cat: Thomson Directory used a several cats to advertise the product, (possibly because of the Tom part of the name). Hundreds of cats appeared in cat food adverts.
Cockerel: Kellogg's Cornflakes. Cockerels have heralded the morning since before Roman times.
Chimps: P G Tips. From the phrase 'chimp's tea party'. This was a piece of entertainment organised by a zoo in less enlightened days...
Crocodile: Wierdly used to advertise Pontins in the 1980s.
Dog: Churchill's Insurance. This has bit them in the backside as more people seem to know Churchill the dog than Churchill the war leader. Why a bulldog though? Because Winston Churchill was portrayed as a bulldog, a sign of defiance. Old English Sheepdogs have been used to advertise Dulux paints (a sign of Englishness?) Toilet Roll and Kitchen Roll (Andrex) has been advertised by labrador puppies (possibly because how else can you advertise it?) A Jack Russell (Nipper) was used to advertise records (His Masters Voice) and now a video and CD shop (HMV). The same breed was also used for John Smith's Yorkshire Bitter. There are course too many dogs to mention in dog food commercials, but the most famous is Henry, who featured in adverts with Clement Freud.
Deer: Babycham. Maybe to have something that slightly resembles a baby.
Flat Eric: A yellow puppet used to advertise jeans.
Frogs: Was used for the American beer Budweiser (there are two versions).
Meerkat: Somehow still advertising comparethemarket.com, despite protestations...
Monkey: Often seen with his pet Johnny Vegas, this toy has been used to advertise ITV digital and PG Tips. P G Tips is obvious (see Chimps above) but there is no particular reason why a monkey was used to advertise ITV digital. Coco Pops also use a monkey, maybe to do the with the colour of the product.
Monster: A honey monster has been used to advertise Sugar Puffs in 1976. Monsters were obviously used to advertise KP Monster Munch, maybe to indicate the huge size of the packet.
Ostrich: Guinness. The illustrator Gillray just painted animals, no rhymme or reason.
Pigs: Natwest Bank. This ceramic pigs were actually made by Wade. It is believed to be a pun on 'pygg' a type of ceramic, in other words a ceramic bank.
Rabbit: A rabbit voiced by Miriam Margoyles has advertised Cadbury's Caramel since the 1980s.
Toucan: Guiness from the 1970, may be because of the black colour. (Illustrations by Gilray, but Dorothy Sayers worked on the campaign).