I assumed you had to pet five different dogs, but after wasting around 20 minutes in Havana and not being able to find a single one I decided to jump on the 'net to look for locations; imagine my surprise when I read you can just pet the same one five times, haha. There are also pigs, chickens and cats in the game, but dogs are the only animal tied to the Abstergo challenges. Even Ubi agrees with the caption, it seems. ;)

such petting
wow

challenge completed: pet 5 dogs .... ummm o.0

Defriended

I don't think we can be friends anymore.

No, you are wrong.

Cats > Dogs

Dogs rule, cats rule slightly less.

You are goddman right

You monster,

Indeed.