The save DC of the ring of animal friendship's charm animal effect is 11.
That's because of this rule:
Magic items produce spells or spell-like effects. For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.
Casting the spell charm animal, a level 1 spell, requires an ability score of 11 because of this rule:
The minimum Wisdom score needed to cast a cleric, druid, or ranger spell is 10 + the spell's level.
The ring's typically created with a caster level of 3 so that's used to determine the spell's effects. For example, the spell, if successful, has a duration of 3 hours because the duration of the spell charm animal is based on the duration of the spell charm person.
Because of the hoops one must jump through to determine a magic item's saving throw DC, most items list that information instead.