Been looking for some trends in Poison and Ground types:
Poison:
Of the sole Poison and Poison/other-type (not other-type/Poison), many have fangs, with most of the minority others with teeth or mandibles.
Most Poison types (and dual Poison/other and other/Poison) are terrestrial, keeping in contact with the ground even when it seems illogical (see Beedrill, Dustox). A few counter-examples are the Ghost/Poison Gastly line, and the only Flying line with a Poison primary type, the Zubat line.
Poison Pokemon have diversity in unusual body structures, compared to the average humanshape or quadruped. These include the snake-like Seviper and Ekans line, Grimer line, Gulpin line, Skorupi line, Tentacool line, Bellsprout line, Spinarak line, Qwilfish, and Budew line.
Ground
The greatest percent of Ground Pokemon are quadrupeds, followed closely by bipeds. The fewest percent are of non-biped or non-quadruped status.
Dual-type grounds often take on the coloration of their other type, such as the Gligar line (note the in-game color for Flying, although one could argue it equally represents its Poison attacks), Nidoran M line, Wooper line, Mudkip line (Marshstomp and Swampert), Camerupt, Wishcash (blue highlights), Torterra, Gastrodon (blue version), and the Gabite line.
Feel free to add onto this list with replies.
