Battling companion is an unofficial name for a genre of anime and manga where the main character, usually a young boy, uses monsters to fight the forces of evil or battle among other characters with monsters of their own. Often (but not always), a character is linked to just one particular monster, which has the role of a sidekick fighting on its owner's behalf or bestowing its powers upon that person. In most cases, the protagonist also travels with a group of friends, each with their own companions, and there is a villain with an ostensibly far more powerful companion than all of the heroes. Storylines therefore tend to be very combat-oriented, with friendships being the secondary theme.
There are multiple variations of the Battling Companion genre. Shaman King, for example, has shamans who can summon spirits to aid them in battle. JoJo's Bizarre Adventure features characters who have "stands"; spiritual sidekicks similar to guardian angels, either to assist in battle or to do things that the person normally cannot. And Beyblade uses tops known as beyblades that contain mythical beasts, enabling them to perform special attacks when they clash with other beyblades.
The first known series of this type is Plawres Sanshiro, but the concept itself originated in 1967 with the sentai series Ultraseven. The main character, Dan Moroboshi, was known to employ "Capsule Monsters" when he was unable to transform into Ultraseven. These he kept in a small metal case on his belt. Moroboshi had three Capsule Monsters, Windam, Mikuras, and Agira, but other capsules were seen inside the box - presumably holding unseen monsters. The capsules were shaped like "time release" capsules with the addition of a small plunger on one end. The idea was to press in the plunger and throw. The concept of grenade-like capsules that contained "shrunken" objects was later employed exactly as depicted in Ultraseven by Akira Toriyama in his Dragon Ball manga as a kind of homage or in-joke to the classic Ultraseven series.