Summon Animals summons 40 animals for 10, while Animal Horde summons 100 animals for 15.

The type of animal summoned depends on the terrain. Provinces with multiple types (like highland mountains) will use the first terrain type appearing in the table below. Some terrains provide substantially more siege and combat power than others, but all are ways to generate a ton of chaff with very few nature mage turns.

Swamps and underwater provinces provide the strongest troops in terms of combat power, probably followed by highlands.

• In swamps: 1/3 Horned Serpent and 2/3 Giant Spiders.
• Underwater: 1/12 Kraken, 1/12 Shark, 1/12 Leocampus, 2/12 Hippocampus, 2/12 Sea Dog, 3/12 Sea Stag, 2/12 Small Shark. The Hippocampus are notable for having a magic Alicorn attack.
• In highland wastes: 1/3 Giant Spider, 1/3 Goat, 1/3 Black Hawk.
• In wastes: 2/3 Giant Spider, 1/3 Goat
• In highlands: 1/3 Great Bear, 1/3 Wolf, and 1/3 Black Hawk.
• In forests: 1/6 Great Bear, 1/6 Moose, 1/6 Giant Spider, 1/6 Wolf, 1/6 Boar, and 1/6 Deer.
• In all other terrains: 1/6 Great Lion, 1/6 Giant Spider, 1/6 Goat, 1/6 Wolf, 1/6 Black Hawk, and 1/6 Deer.

Animal Horde is much more gem efficient than other ways to summon the various units it generates that can be summoned another way. For example, you'd need to spend 24 and 6 mages turns casting Summon Horned Serpents to get as 30 Horned Serpent, compared with 33 from a single 15 cast of Animal Horde in a swamp, which comes with an extra 66 Giant Spider.

All summoned animals are undisciplined. Most have high combat speed, and very low protection and magic resistance, and will lose to modest province defense unless you bring overwhelming numbers. Nonetheless, there are plenty of use cases for tons of chaff that can be useful in various chaff roles, either as evocation bait or, with enough late game buffs, as blockers in an army.

Source: https://illwiki.com/dom5/user/loggy/misc