Different animals have different collective nouns;

• A murder of crows
• A school of fish
• A congress of owls
• A misery of redditors

I did some poking around and it looks to me like pack generally refers to animals that hunt and herds are generally subject to predation by pack animals. The collective benefits of herd behavior vs pack behavior appear to also be distinct, with packs demonstrating a higher degree of social structure or hierarchy.