Surprise! A group of frogs is called an "army". During the breeding season when frogs are easy to spot, they can form huge groups. This is probably the reason that a group of frogs is called an army or a colony. Collective nouns often show great imagination; a barrel of monkeys, a murder of crows,a parliament of owls, etc. We all know of herds and flocks but some of the collective nouns are very unusual.

More Info: en.wikipedia.org