To attack, or not to attack?

Question: Assume you control a Jackal Familiar which is enchanted with Furor of the Bitten. You also control a second creature (that has nothing special to it.) Are you forced to attack with both creatures to make the Jackal attack?

Jackal Familiar can't attack or block alone. Furor of the Bitten says that the enchanted creature must attack each turn if able. Because of that last part, many players will intuitively answer that Jackal doesn't have to attack if nothing else is attacking, because it's not able to do so alone. Since it's not able to attack, it doesn't have to.

While this sounds logical and intuitive, it's actually incorrect. In fact, you have to attack with both creatures (if both are otherwise able to) in order to fulfill the requirement imposed by Furor of the Bitten. In a way, the combination of Jackal's and Furor's abilities causes a mandatory two-creature attack on each turn, if both of those creatures are able. (If Jackal is your only creature, or your other creatures can't attack for other reasons, eg. summoning sickness, then Jackal doesn't attack.)

This is one of the more obscure parts of the rules of the game, which often surprises players.

There is a rule that states that when declaring attackers, the player must declare them in such a way that fulfills as many requirements as possible. Since declaring two attackers (in this case) fulfills more requirements than declaring no attackers, the former is mandatory.

Note that this applies only to declaring attackers. If there are costs that have to be paid (eg. mana costs) in order for creatures to attack, those don't have to be paid. For example, assume that your opponent controls a Sphere of Safety.

In this case you are not forced to pay the mana cost required to attack, and thus your creatures are not forced to attack.

The exact rule that states this is the following:

508.1d The active player checks each creature he or she controls to see whether it’s affected by any requirements(effects that say a creature must attack, or that it must attack 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 attackers is illegal. If a creature can’t attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed.