Soto's Saber Pack was a group of saber-toothed cats that appeared in the first movie. The herd originally consisted of 10 sabers who lived together; but because of an attack by a human tribe, half of the pack was hunted down and killed sometime before the migration shown at the beginning of the movie. Afterwards, the only known members were Soto, Diego, Zeke, Oscar and Lenny.
As the movie continues, Soto sends Diego to bring him the son of the leader of the tribe so that he can have his revenge on the humans. The next morning, the sabers attack the human camp and Diego eventually finds the baby, but he's stopped by the baby's mother who grabs her son and runs away with him. Diego traps her in the edge of a waterfall, so she's forced to jump into the water while holding tightly onto her baby. After meeting again with the sabers, Diego is being told by Soto to get the baby and take him to Half Peak.
But other two mammals named Manny and Sid had already found the baby and they were planning to deliver him back to the humans. Diego joins them as it's the only way to be able to stay close to the baby and track him down as Half Peak was very close to Glacier Pass. As they journey through, in the night of the first day of traveling, Zeke and Oscar visit Diego to tell him how impatient Soto's getting, so Diego tells them to go and tell Soto that he'll take the baby, and a mammoth (referring to Manny).
They are briefly referenced by Diego in the fourth film, when he tells Shira that he knows how hard it is to leave a pack.