We used the male megaloceros to tank the alpha dragon boss on island map. Get a journeyman + saddle and then use veggie cake to keep the hp up periodically during the battle ( I think I used 10) run directly to the dragons testicles and schwhale on its balls with your horns a few times. Push toward the tail and you will notice you are locked in, then you and your raid is clear to start pump action shottty on it. Dragon might get unlocked one to three times, spread rexes and raid during air phase, more than one megaloceros tank recommended in case one goes down somehow.
They are very good combat and transport mounts. The males can attack, which allows them to gather thatch very efficiently. The females have no attack, but are a little faster, as there are no horns to weigh them down. There's also something satisfying about riding a moose. WHEEEEEEEEEE