There's a GM ruling to be had here, depending on the order they choose to apply effects. At my table, Yes, the creature can take and benefit from the feat, depending on the circumstances.
Taking the feat: The prerequisites are pretty straightforward. It's fairly clear you can take the feat if you have at least three natural attacks.
Benefiting from the feat: This is a little harder, and where the GM call comes in, but here is what I consider the "common sense" call. Normally, a creature with only primary natural attacks doesn't gain any benefit from Multiattack because they have no secondary natural attacks to reduce the penalty on. However, many creatures are intelligent enough to use weapons. When you make a full attack with one or more manufactured weapons, you can still add each of your natural weapons (except usually claw or slam attacks that use the hand you're holding the weapon in) to that full attack in addition to getting your full complement of normal attacks, but those natural attacks are treated as secondary attacks regardless of their normal status.
Your example creatures with two claws, a bite, and a gore could still use the bite and gore (and possibly a claw) while taking a full attack with a weapon. Depending on his BAB, he might be able to take multiple attacks with a weapon or two and still get in two or three of his naturals as well. In these cases, the Multiattack feat will offset three points of penalty to each natural weapon attack, changing them from -5 to -2. If the damage difference between his claws and his weapons is significant, this can be a very advantageous move.
Two other uses for using weapons in addition to natural attacks: reach weapons and ranged weapons. Using a longspear can double your reach, while allowing you to retain the ability to attack and threaten the adjacent squares as well, thanks to your bite and gore attacks. An archer or crossbowman could still make two attacks into his melee range as part of a bow full attack, and still threaten melee as well. Just note that your ranged attacks still provoke attacks of opportunity.