Camarasaurus

Archnemesis. The Camarasaurus deals double damage to Apatosaurus, but takes two and a half times damage from them. The Camarasaurus has disadvantage on saves from the Apatosaurus but has advantage on attack rolls against them. They are pushed an additional ten feet away from them when pushed/knocked back by them, which makes them have disadvantage on attack rolls after that. If the Camarasaurus is tamed and sees an Apatosaurus, this happens as well and they won't listen to their owners, whether or not the Apatosaurus is tamed by the same person or not.

Intolerant. The Camarasaurus hates Apatosaurus, but they get extra effects from the following creatures as well: Dodo, Pteranodon, Brachiosaurus, Diplodocus. The Camarasaurus has advantage on attacks against these creatures and deals an extra three damage dice to them. Upon seeing these creatures, it must succeed on a DC 7 Intelligence saving throw, with advantage if tamed, or attack them. This happens every time they look at them, if they look away for a second and look back, they must roll the save. If both the Camarasaurus and the disliked creature are tamed, the Camarasaurus doesn't need to roll the save but doesn't care if it harms it.

Rage. Upon seeing an Apatosaurus, the Camarasaurus may immediately move 30 feet towards it and make an attack if in range, and it deals two extra damage dice for 1 minute after it lets its eyes off of it. While enraged this way, the Camarasaurus attacks anything it thinks it can easily best, and has 10 temporary HP. It also has immunity to the frightened, charmed, and paralyzed condition, and suppresses all levels of exhaustion while raging.

Multiattack. The Camarasaurus attacks twice when it takes the attack action.

Tail Sweep. Melee Weapon Attack: +12 to hit, reach 15 ft., 2 targets. Hit: 18 (2d8 + 9) bludgeoning damage and the target must succeed on a DC 19 Strength saving throw or be knocked prone. A clever character can jump over and onto its tail with a DC 23 Dexterity check and go for a ride. Doing so removes all damage and makes the Camarasaurus lose you and lose interest on a DC 14 Animal Handling check. Then you might be able to tame it.

Push. Melee Weapon Attack: +12 to hit, reach 25 ft., 1 target. Hit: The target is pushed 25 feet away unless it succeeds on a DC 15 Dexterity or DC 20 Strength saving throw.