The Scarab Underwalker is a rare pet that F2P and Subscribed players can earn as a reward on Yavin. The pet only comes from the Revanite Captain as kill loot.
The Revanite Captain shares a spawn with Commander Jenarian, this means that if you find Commander Jenarian at the spawn point, you must kill him for the Revanite Captain to spawn. You can find the Captain/Jenarian at the center-east of the Training Grounds on Yavin, specifically at the training arena at X: 2375, Y: 675.
Commander Jenarian has vastly more HP, meaning he takes longer to kill then the Revanite Captain. The respawn timer is only a few minutes so it should not take long to have the Revanite Captain spawn after killing Commander Jenarian.