OKay. A few ideas not related to any campaign of mine:
1) Running African Warriors, built around the Zulu infantry - they carry long spears, long shields, and run massive distances. They have trained extra large baboons to follow them. Each baboon is tied to a specific warrior, and carries much of the warrior's gear. The baboons avoid actual fighting, but are territorial guards when the armies camp at night.
2) I think halflings should be able to befriend talking birds. Halflings with singing parrots are just plain awesome. They should also use the parrots as food in harsh times - but only eating the parrots that don't have a wide vocabulary. Any parrot that can say "hey, don't eat me!" is spared.
3) Aquatic elves use jellyfish. They can't really "train" them, but they have learned ways to influence the movements of friendly strains of jellyfish, using them as doors and walls in their aquatic homes. Also, the aquatic elves keep cuttlefish as pets, and the more artistic elves use the cuttlefish's chameleon ability in their performances. Elven generals use the cuttlefish as a signaling ability in springing ambushes, and use the ink to effect hasty escapes.
4) I had an arctic campaign idea where the entire world was covered in ice. "The mammoth people" were halflings who had domesticated woolly mammoths, and used these mammoths basically as cranes and bulldozers. And of course, each mammoth could carry a lot of stuff. Each halfling family was tied to one mammoth, and before new families could get married, they had to have their own mammoth, usually a gift from the bride or groom's parents.
5) I think the idea of Grippli domesticating Carrion Crawlers is one of the coolest things in the world. Imagine the little frog people riding giant centipedes into war, and then hopping up into the trees and using poisonous darts against the archers while the crawlers paralyzed the front ranks of the enemy. Am I the only person who thinks that is worth basing a campaign around?