First Appearance: WB2014 Digdoug - Episode 3
Felgercarbs are Heavy Units employed by Numloch. They have horns which they can use to dig into enemy city's walls. Perhaps the similarity of 'carb' to 'crab' implies that they have a form similar to a large crab in the Real World.
Real World References
Felgercarb is an expletive from the 1970's Battlestar Galactica, used in place of *ahem* "crap".