If given the chance, Argo could probably make a ton of friends and not just horse friends either. Heck, there are probably many people out there who'd love to have Argo as their pet (okay, maybe "pet" isn't the right word, but you get the point). On the show, Argo has come across many people both good and bad. From simple villagers to demons, Argo has had a chance to meet every type of person in the known world. But the ones she is most familiar with are Xena, Gabrielle, and even Joxer. So they are definitely ones that are most often seen with Argo.
As horses go, however, we don't know about many other horses that Argo knew. Argo did get to meet a stubborn little donkey in "A Solstice Carol" named Tobias but the donkey was only around for that episode so there wasn't much bonding between Argo and Tobias.
The only other horse that was present on the show in many episodes (and even had a main role in one of them) was Amber, Gabrielle's horse. Though their relationship was short lived due to an unfortunate 25-year sleep, Amber was an intelligent horse who learned from Argo.
Amber first appeared in "Animal Attraction", when Gabrielle, Xena, Joxer and Amarice came across a horse caught in a hunters trap. Xena was able to free her but not without the help of Gabrielle who calmed the horse down while Xena released the trap from its leg. Gabrielle immediately fell in love with the horse from the moment she saw it and she knew that she wanted to keep her. Gabrielle named the horse Amber, though the horse's name was never revealed in the show. It was mentioned on the Official Xena Universal Studio's site (that is no longer in existence) that the horse's name was Amber.
It took a while for Amber to trust Gabrielle. Even with Argo's encouragement, Amber still wouldn't trust Gabrielle. It wasn't until after Gabrielle pretended that her leg was hurt (just like Amber's was) and was able to come to a level of understanding between them, that Amber finally learned to trust Gabrielle.
After that, Amber was seen in quiet a few episodes: "Them Bones, Them Bones", "Purity", "Back In the Bottle", 'Punch Lines", "Eternal Bonds", and "Amphipolis Under Siege." Whenever Xena was riding Argo, Gabrielle was usually right beside her, riding Amber. But after Xena and Gabrielle's 25-year sleep in the ice cave, Amber wasn't seen again after that. She could have died of old age just as Argo had but it was never mentioned what happened to her.
After Amber, Gabrielle was scene riding other horses, from a white one in "The Rheingold" and "Dangerous Prey", a brown one, much like Amber, in "Legacy" and a speckled gray one in "Old Ares Had A Farm." So Argo (or rather Argo II) probably had other horses she could be friends with besides Amber.