With the Add-On Pack you get access to 28 cards: 24 additional creature cards (12 new creatures with duplicates) and 4 additional Mindbugs (required to play the 4-player mode).
List of creature cards:
Bugserker (x2)
Count Draculeech (x2)
Creep from the deep (x2)
Ferret Pacifier (x2)
Froblin Instigator (x2)
Goreagle Alpha (x2)
Hamster Lion (x2)
Hungry Hungry Hamster (x2)
Hyenix (x2)
Majestic Manticore (x2)
The Lurker (x2)
Turf the Surfer (x2)
Mindbug was co-designed by Richard Garfield (of Magic: The Gathering, Netrunner, and King of Tokyo fame). Mindbug is a fast and clever dueling card game in which the strongest and best card played at the wrong time may result in disaster (for yourself).