A Familiar is a creature that has been tamed by or given to a player to used as a pet or a mount. Riders of Icarus puts a great amount of emphasis on Familiar collecting and mounted/pet combat. 

Each zone in the game contains specific creatures that can be tamed. Some of them require special items to be in the player's inventory, others may require a specific achievement or misson to be completed (game is still in beta, so subject to change). Players will earn taming points while they play that are requred to tame Familiars. Each one requires a specific amount of points. Each failed tame attempt will cost points, so learning to do it properly is key or players will find themselves too low on points to tame the next creature they encounter, even if they have the required items. For more information on how to tame, see Taming

Below is a list of Familiars by zone:

Brakarr Forest

Hakain's Crossing

Hakanas Highlands

Sea of Hakanas

Parna's Coast

Tritael Rift

Cloying Wastes

Exarahn Badlands

Exarahn Badlands - Part II