All Animal Companions save as the creature type in the Creature Guidebook, except that the creature’s Save vs. Sleep Charm is upgraded one step. This represents the increased loyalty and empathy that are synonymous with Animal Companions. So, if an animal has a Poor Save vs. Sleep/Charm, once it becomes an Animal Companion, it has an Average Save vs. Sleep/Charm.
Note that this rule applies both to a Ranger’s Animal Companions, as well as ANY animal that a character has befriended and been with for a reasonable amount of time. Rangers simply get the ability to train such animals freely at a fixed level while other classes must invest the needed skill points into Animal Handling or a similar skill.
Once an Animal Companion has been separated from its master for over 1 year, it loses its bonus to its Save vs. Sleep/Charm, though it does not forgot its master or cease to consider him or her a close friend and ally unless it parted ways with its master on hostile terms.