Rex

Rexes are enemies in the Super Mario Bros. series, first appearing in Super Mario World. They are purple dragons with small wings that walk around, taking 2 jumps to defeat. Despite having wings, they do not have the ability to fly. They do not appear in any mainline Mario Bros. games after Super Mario World, though has appeared in several spin-offs and other media.

Super Mario 64 Manifestation
According to some reports, Rexes can appear in Super Mario 64, most commonly in grassy levels such as Bob-omb Battlefield. They can reportedly be based off of the Yoshi model, or have an entirely custom model. They usually follow their behavior from Super Mario World, though can reportedly have new behaviors such as breathing fire.

Some have speculated that Rexes are a remnant of Super Mario FX, a supposed precursor to Super Mario 64 that would have been much more similar to Super Mario World, though this is unconfirmed. Others have simply proposed that more enemies from other Mario games would have been present, appearing to explain other unused enemies such as the Bloobers and Angry Sun, instead of new enemies such as Spindrifts and Mr. Is.