Loathsome Catoblepas doesn't tell on it's card about consequences, so I felt it's important to make sure, what happens if you cannot block this creature in any way.
Loathsome Catoblepas creates a requirement for blocking. This means that if the defending player has at least one creature that is able to block the Catoblepas it must be blocked by at least one of those creatures (if the defending player wants to block with more they can as long as one is). If there are legal blockers and none are declared as blockers it is an illegal block and the game will rewind so that a legal block can be declared. If there are no creatures able to block the Catoblepas the ability does nothing.
509.1. First, the defending player declares blockers. This turn-based action doesn't use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 717, "Handling Illegal Actions").
509.1c The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed.