Some cards, such as Loyal Pegasus and Mogg Flunkies feature the rules text
Card name can't attack or block alone.
For attacking, this is unambiguous; the creature can only attack if another attacker is also declared.
For blocking, which of the following is the case?
- The creature can only be declared as a blocker if blocking the same creature as another blocker
- The creature can only be declared as a blocker if any other creature is also declared as a blocker, regardless of what that creature is blocking