Guts has giant paws and long arms so probably upright bass
Grendel purrs super loud and doesn't like to put effort into anything, so I gotta say, kazoo

In fact, a study from 2012 indicates that felines have a complicated system to keep track of social and economic data such as the number of group members, prey animals, property, or debts (that is, accountancy). However, the primary use they have for this particular skill is...

...counting sheep...