The Maw

Yums are the first creatures that Maw will eat and they look exactly like the achievement icon. They appear in every level, except level 8 'Planet Maw' where Maw doesn’t actually eat anything. They can be found in the tall, slim green trees, rocks, and mushrooms. They also appear in the flower patches with the knee-high grass clumps and white and purple flowers. Make sure that you run all over these patches so that all of the Yums pop out of the ground; it is very easy to miss one.

Yums can also be found in the ‘Brute Force’ and ‘River Redirect’ DLC levels. The numbers of Yums found in each level are:

Level 1 ‘Crash Site’: 42
Level 2 ‘Bulb Valley’: 37
Level 3 ‘Chaindown’: 22
Level 4 ‘Dirtlands’: 45
Level 5 ‘Looferland’: 48
Level 6 ‘Mesas’: 14
Level 7 ‘Beetull Smash’: 28
‘Brute Force’ DLC: 65
‘River Redirect’ DLC: 33

If you are going for the 100% eaten achievement, and do not need to repeat a level then you will get this in level 6. However, your progress carries over every time that you replay a level; I got this on my second playthrough of level 5.