Squirtle evolves into Wartortle at level 16. One of the major differences between Squritle and Wartortle are the tails. In the anime version of Pokemon, Ash has a few Wartortle entries:
EP060: Wartortle, the Turtle Pokemon. The evolved form of Squirtle. Its long furry tail is a symbol of its age and wisdom.
EP106: Wartortle, the Turtle Pokemon. The evolved form of Squirtle. A highly sought after Pokemon because its long fur-covered tail is said to bring good luck.
DP077: Wartortle, the Turtle Pokemon and the evolved form of Squirtle. Its furry tail is very popular as a symbol of long life.

So, according to all of the entries, Wartortle's tail is significant because it's a symbol of wisdom, it brings good luck, and it is a symbol of long life. We see May's Squirtle evolve into Wartortle in the anime episode Staging a Heroes' Welcome!.

Its design appears to be a stylized sea turtle with features of tortoises. Its feathery tail is a reference to the Japanese legends of the minogame, a turtle which lived for 10,000 years and grew a tail made of seaweed. The use of its tail to store oxygen seems to be a reference to certain turtles that breathe through their cloaca, placed near the tail.


Warortle evolves into Blastoise at level 36. Blastoise is the final evolutionary form of Squirtle. However, if you give Blastoise a Blastoisinite to hold during battle, it will Mega Evolve into a mega stage. Blastoise is the version mascot of the Japanese and international versions of Pokemon Blue, appearing on the boxart of both.
EP060: Blastoise, the Shellfish Pokemon. The evolved form of Wartortle. Blastoise's strength lies in its power, rather than its speed. Its shell is like armor and attacks from the hydro cannons on its back are virtually unstoppable.
EP248: Blastoise, the Shellfish Pokemon. Blastoise uses the Hydropumps on its back to fire out extremely powerful blasts.
AG092: Blastoise, the Shellfish Pokemon. Blastoise is the final evolved form of Squirtle. It can launch powerful blasts of water from its water spouts.

We see Squirtle evolve all the way to the last version, Blastoise from Pokemon: The Origin files with Blue. Blue sent out his Blastoise in File 4: Charizard during his battle with Red. It defeated Red's Persian and Jolteon, but lost to his Charizard.

Its design appears to be a mix of a sea turtle and a tortoise, further mixed with aspects of a tank. Blastoise's cannons may have been derived from the functioning tubes found in mussels and some other shellfish, thus its species name.

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