The "virtual pets", which have been created using text-to-speech technology, aim to raise awareness of the charity's new social network for pet lovers, Allaboutpets.org.uk.
Users can create their own talking pet by choosing from a selection of virtual dogs and cats, or by uploading a picture of their own pet.
Technology from avatar specialists Oddcast allows the virtual pets to be customised with accessories and animated backgrounds, while uploaded images can be fitted with an appropriate mouth.
Users can then type a message for their talking pet to say, or choose a pre-recorded message with a range of different voice options.
The final talking pet can be sent to friends or uploaded into a Facebook profile.
It is hoped that the viral will help drive traffic to All About Pets and boost sign-ups prior to its official launch in August.
Debbie Curtis, head of communications at The Blue Cross, said: "This viral campaign is fun, entertaining and a perfect way to reach pet owners who will enjoy being part of our new online community.
"It has been exciting to work on something so innovative, creative and unique, and we are confident that it will help us to attract even more members to the site in the run up to its official launch."
The ads will feature an interactive talking dog and users will be encouraged to type in messages for the dog to say. Users will then be directed to click through to the talking pets microsite.
One of the ads appearing on AOL will combine text-to-speech functionality with page-scraping technology to create a "virtual newshound" that can read the headlines of the host's webpage in real time.
The concept of the viral campaign and its promotional MPU's were devised and created by WWAV Rapp Collins London.
Digital PR agency Hot Cherry will promote the online viral campaign and the new social network across a network of bloggers, online communities and other websites.