Does Beast Below need a friendly target to work?

Yes, Beast Below requires valid targets to put its "When you play" trigger on the chain, but you can still play the unit itself even if you cannot fulfill those targeting requirements.

According to the RiftJudge FAQ for Beast Below (FAQ #466 and #9370):

• Targeting Requirement: The ability "return another friendly unit and an enemy unit to their owners' hands" targets both a friendly unit and an enemy unit. To put this triggered ability on the chain, you must have valid targets for both.
• Playing the Unit: You are permitted to play Beast Below as a unit even if you do not control another friendly unit (or if there is no enemy unit to target). In this scenario, you simply do not put the "When you play" trigger on the chain because you cannot satisfy the targeting requirements.