Ekans, Arbok, Seviper, Huntail, Onix, and Steelix are Snakes because they have a Snake-like body. Huntail, Onix and Steelix may sound funny for snakes, but their body-type fits: Huntail is a Sea Snake, and Onix and Steelix are Ground Snakes.

Squirtle, Wartortle, Blastoise, Torkoal, Turtwig, Grotle, and Torterra are Turtles. I wouldn't classify the Squirtle family as a Sea Turtle, though. Biologically it can't hold that name since it doesn't have flippers (although it is a water type).

Treecko, Grovyle, Sceptile, and Kecleon are Lizards. Charmander, Charmeleon, and Charizard aren't exactly lizards, per say. They actually fall under the Monitor Lizard or Pseudo-Crocodile category, since they are bulkier than regular lizards.

Totodile, Croconaw, Feraligatr, Aron, Liaron, and Aggron fall under the Crocodile category. The Aron family falls under this category instead of the Dino category because all of their evolutionary stages have the trade-mark Crocodile back texture (that bumpy back with a low-to-the-ground tum is a Crocodile build)

Aerodactyl, Omanyte, Omastar, Kabuto, Kabutops, Lileep, Cradily, Anorith, Armaldo, Cranidos, Rampardos, Sheildon, and Bastiodon are Dinos.