In the Monster Manual, a homunculus is a construct created as an intelligent companion, sharing a telepathic bond with its creator. In Eberron, homunculi possess additional abilities that aid their master in specific ways. Eberron: Rising From the Last War includes stat blocks for two: expeditious messengers and iron defenders. Both are exactly what their names suggest, and both are fairly low-challenge and narrow in their applications.
The expeditious messenger is a flying construct made to convey messages. Although it can’t hover, it has little reason to land, since its flying speed is more than double its ground speed. It has proficiency in Acrobatics and Stealth, equipping it to elude interception via creative aerial stunts. (Mechanically speaking, that +5 to Acrobatics checks is important because it helps the expeditious messenger escape a grapple.) It also has the Flyby trait, so it never needs to Disengage in order to keep a pursuer from knocking it out of the air.
It has the ability contour of a shock attacker: high Dexterity and not a lot else going for it. But calling it an “attacker” at all is misleading, because it has no reason to be aggressive. Most of the time, it stays out of reach of other creatures, in the interest of speed and efficiency. If an enemy tries to grab one, it makes one Bite attack as a deterrent before flying off, immune from opportunity attack.
Other than that, it takes either the Dash action or the Dodge action by default, depending on its environment: If it’s being chased by an invisible opponent, if it’s passing through a heavily obscured area (e.g., thick fog) or if it’s subject to multiple enemy attacks in a round, it Dashes. If the air is clear or lightly obscured, there’s only one foe taking shots at it, and that foe is visible and gets only one attack per round, it Dodges. Anytime it comes under attack, it tries to get out of sight, zigging and zagging through whatever cover is available. As a Tiny creature, it can zip through an opening as small as 2½ feet wide without even slowing down, or through a 1¼-foot opening by squeezing.
If the expeditious messenger is conveying a message, it’s singleminded in its determination to deliver that message to its recipient. But if it comes under attack on its way back, it dives for cover upon taking even a single point of damage, telepathically informing its master of where it’s holed up, and doesn’t come out again until the heat is off.
The iron defender is a loyal guard beast with the ability contour of a brute. Its function is to place itself between its master and any attacker. It has the skills of an ambush predator, but I think its nature is more that of a guard dog; that’s certainly what its illustration suggests. Its Keen Senses give it broad advantage on Wisdom (Perception) checks, whether using sight, sound or smell. It’s not smart enough to be particularly good at target selection, but because of its Telepathic Bond, its master can tell it what opponent(s) to prioritize.
Generally, its modus operandi is straightforward: Bite and don’t let go. (Interestingly, as worded, the iron defender can have one target grappled and Bite a different target at the same time. I think this may be a mistake, but it’s not in the errata, and it’s plausible if you imagine that the iron defender is heavy and can pin an enemy down with its paws and claws.) Strategically, however, I imagine that the iron defender’s role is to delay attackers while its master escapes from the scene. Until they’re safely away, the iron defender engages whichever enemy has the best chance of catching up with its master, instinctively backing itself into a choke point if possible so that no one can simply run around it. Once its master is in the clear (which they can inform the iron defender of telepathically), it withdraws itself, Dodging as it goes (it lacks the Intelligence to Disengage). Its loyalty and obedience are absolute: While defending its master, it fights until it’s destroyed.
Next: living spells.