From the Super Mario Wiki
Goombrats are a species of Goomba that debuted in the Wii U title New Super Mario Bros. U. They are short, are lighter brown, and resemble persimmons more than mushrooms. They have different facial features, and the black stems on their head are similar to tomato leaves. They also turn when they reach edges, which is similar to the behavior of red Koopa Troopas. The enemy's name is a composition of "Goomba" and "brat". Also their teeth point down, when regular Goombas teeth point up.
These sub-species first appear in the sky level of Acorn Plains, Mushroom Heights and one of the levels in Challenge Mode, The Goombrat Stomp.
In New Super Luigi U they are significantly more common.
Goombrats are set to appear in Mario Party 10 in a minigame featuring Goombas and Galoombas.
Names in other languages
||Conjunction of "Goomba" and "niño", Spanish word for male child.
||From Goomba and "Gamin" which means a child.
||A conjunction of Goomba and brat.
||Possibily "Little Goomba"
||"Goomba" with the not-so-used suffix "ito", meaning small.