Can you play Beast Below with no friendly units?

Yes, you can play Beast Below even if you do not control any other friendly units.

According to the RiftJudge FAQ (#466, #9370, #9889):

• Playing the Unit: You are permitted to play Beast Below as a unit even if you do not control another friendly unit.
• Triggered Ability: Its "When you play me" ability requires you to target another friendly unit and an enemy unit. Because valid targets must exist for all required targets to place an object on the chain, if you do not control another friendly unit, you simply do not put the triggered ability on the chain.
• Result: The unit enters the battlefield normally, and the ability—which would have returned units to their owners' hands—never triggers or enters the chain, so your unit remains on the battlefield.