Guild Wars 2

What constitutes a companion? I want to use ranger runes, but not sure if a companion is only a pet. Are companions the green dots on mini maps? If I’m a necro, would flesh golem be considered a companion?

Unless they’ve changed it, a companion is literally anything. Again, they may have totally changed it, but mini pets even counted as companions. You got the bonus when you just had a mini out.

For the purposes of triggering the bonus damage (#6), the game considers summoned NPCs to be companions; this includes illusions, minions, and those produced by sigils (e.g. Sigil of Demon Summoning) or various consumables.
However, you lose the bonus if your animal companion dies.

Anomaly:

The bonus is also applied if the player has a miniature out or has used a summoning skill from a gizmo, e.g. via Summon Familiar (bat).

(from GW2 wiki)

http://wiki.guildwars2.com/wiki/Rune_of_the_ranger

The “Notes” section has all the information you need! Basically, companions are summoned NPCs or pets.