Although yetis are not uncommon, they live solitary lives until they mate. After mating, the male and female will live together until death. Yetis are highly territorial, and will ferociously defend the deep mountain caves in which they live.
A Yeti invaded Queen Icebella's Realm shortly before Mordack attacked Castle Daventry. Queen Icebella sent Graham to defeat the yeti, which he did by throwing a custard pie into its face. Without its sight, it fell from the high cliffs of the mountains.
There is only one yeti -- after being outwitted by Alexander, he moved to Queen Icebella's Realm and took over her prized crystal cave.
The King's Quest Companion calls the Abominable Snowman in KQ3 a Yeti, and the Yeti in KQ5 an Abominable Snowman in various sections (An Encyclopedia of Daventry, and the hint sections). The second edition explains they are the same race. There is nothing specific to suggest that the one seen in each game is the same character or not, but it does point out that they are a common species in the high altitudes.