Pippis are small bird-like creatures that you can find in Biomutant, and are needed for a trophy / achievement called "Flip-flap". This achievement requires you to capture a total of 20 pippis, so how do you get them?
Pippis can spawn randomly anywhere on the world map in large groups. When you get close all but one of them flies away, so you can only capture one pippi from each flock of birds. They also have a fixed spawn point at the location of the NPC called Lobo in the far north of the map in the Kluppy Dunes region.
The location where you find Lobo will either have 1 or 2 pippis that you can capture. In contrast to a lot of the other animals there is no respawn timer, so you can go there to find 1 or 2 new pippis each time you enter the area.
Since they only spawn one or two at a time, getting all 20 can be a time-consuming / boring challenge. But you can make it a lot faster by fast travelling back and forth between this location and another one since the pippis will respawn each time you do this.
You do not get a notification when you capture a pippi, and they will just disappear when you capture it without giving you anything. However, the game keeps track of how many you have caught, so you get the trophy / achievement after getting your 20th pippi.