What was the first new Pokemon to appear in the anime?

This really depends what you mean by "the anime"
Pikachu was the first pokemon ever seen, then the rest of the 151 "original" (149 up to the 1st film then 2 more in the film.)
The first of the "next lot" was cyndaquil number 155 which first appeared in season 2.
The first new pokemon to be actually seen in the film, was Mew2, which was a clone of Mew.
A list of pokemon 141-160 is here...
https://en.wikipedia.org/wiki/Cyndaquil#Cyndaquil