Calculating the XP treshold for a party with “monster” companions

Suppose that a member of an adventuring party has a companion. I can think of the following broad categories …

  • Something that came with his/her class (familiar, beastmaster companion)
  • Part of a spell (summoned/conjured creatures, raised undead, Homunculus, awakened something, etc.)
  • Purchased mount (war horse, hippogriff, etc.)
  • A bound golem

… then are there cases (and perhaps there are others besides the ones I could think of) in which I should I calculate the XP treshold (DMG, page 82) differently (increasing the treshold), and if so, by how much?

Personally, I feel (but I’m quite inexperienced) that companions that originate from classes and spells are already calculated into the character level of the XP difficulty matrix. And that mounts may not be powerful enough to unbalance the encounter difficulty. But a party member that brings a golem, might be a different story.

I’m fairly sure that there is no rule for this, if so, then suggestions are welcome too.