To answer your questions…
- Speed generally determines who gets to attack first. If two creatures have the same speed stat, the higher rarity strikes first. If two of the same kind of creature face one another and are the same level (i.e. Lv. 10 Velociraptor vs. Lv. 10 Velociraptor), then the first player to hit the attack button first will go first.
Other factors that determine your ability to attack first (or at all) include short-term invincibility shields, Cloak/Evasive Stance, attack-reducing moves, and stunning attacks. Take time to familiarize yourself with your creatures, their abilities, and study up on the creatures that your opponents seem to use regularly.
- What kind of creatures are you facing that haven’t been unlocked or made available yet?
All 112 creatures currently in the game can either be found in the wild, obtained via Strike Towers, or incubators of varying rarity (depending on your place in the arena, in some cases).
- A sufficiently leveled Diplocaulus Gen 2 (or even Gen 1) could kill a T-Rex, but it’s not a great match-up for T-Rex to take on regardless.
It takes time and effort to build a decently strong team—never fear, you’ll get there! And there are plenty of folks on this forum who’re happy to help give advice or offer suggestions as to how to build a stronger team.
Best of luck, and hope this helps!