Chronologically the second of the Squirtle belonging to the named trainer (May) who appears in a large number of episodes. Initially, he is distinguished by a very tearful and timid character, and with time and with subsequent fights, he becomes bolder. He mainly occurs in “competitions”, where important thing is in what style pokemon will defeat the opposing Pokemon (while in normal fights, only effectiveness matters). As part of the main series, while May accompanies Ash, Squirtle remains in her basic form, during her guest appearance on the show sometime later, we can see that he has evolved into Wartortle in the meantime.
Below is a list of the episodes in which this Pokemon appears – the first number is the overall episode number, the second is the episode number within the sub-series, AG is Advanced Generation, DP is Diamond & Pearl, PK is a special episode (focusing on Pikachu). Episode numbers according to the Bulbapedia.
407 – AG133 – The Right Place and the Right Mime – Debut.
408 – AG134 – A Real Cleffa-Hanger
411 – AG137 – Hooked on Onix
412 – AG138 – Rough, Tough Jigglypuff
413 – AG139 – On Cloud Arcanine
414 – AG140 – Sitting Psyduck
417 – AG143 – The Saffron Con
418 – AG144 – A Hurdle for Squirtle
425 – AG151 – Weekend Warrior
426 – AG152 – On Olden Pond
430 – AG156 – From Cradle to Save
431 – AG157 – Time Warp Heals All Wounds
433 – AG159 – Off the Unbeaten Path
434 – AG160 – Harley Rides Again
435 – AG161 – Odd Pokémon Out! – Small part.
440 – AG166 – Curbing the Crimson Tide!
441 – AG167 – What I Did for Love!
448 – AG174 – New Plot, Odd Lot! – Small part.
449 – AG175 – Going for Choke!
454 – AG180 – May, We Harley Drew’d Ya! – Squirtle visible only on the screen with the information about May’s Pokemon.
455 – AG181 – Thinning the Hoard!
456 – AG182 – Channeling the Battle Zone!
457 – AG183 – Aipom and Circumstance!
458 – AG184 – Strategy Tomorrow – Comedy Tonight!
463 – AG189 – Gathering the Gang of Four! – Ash’s Squirtle also appears
466 – AG192 – Home is Where the Start Is!
543 – DP077 – Staging a Heroes’ Welcome! – Appears as Wartortle.
PK17 – Pikachu’s Island Adventure
M08 – Pokémon: Lucario and the Mystery of Mew
M09 – Pokémon Ranger and the Temple of the Sea
Movie TV Special 2 – Pokémon: The Mastermind of Mirage Pokémon