Pathfinder wasn't mentioned at all in the OP, so anything from it might not be allowed.
We only hit up Pathfinder if everything short of Houseruling fails.
Pathfinder wasn't mentioned at all in the OP, so anything from it might not be allowed.
Why not just use the stats for the regular animal, but tag it as a Construct instead of Animal/Beast? Use the same immunities/weaknesses as the Warforged character.
Essentially, the fact that the companion is a construct is 90% flavor. So just reskinning the regular animal will work for the vast majority of cases.
Then just use the Construct rules as necessary when it would actually make a difference. It's sort of like how for the most part you treat a Warforged as a regular person, with the mechanical difference only appearing in special cases.
There are rules for more powerful Animal Companions right in the core books. They compensate for the more powerful creature by effectively lowering the Ranger or Druid's effective level, for purposes of advancement benefits of the Companion.
For example, the Druid in our group has a tiger as his animal companion. By the book, he gets the companion benefits of a Druid that's six levels lower. That means less Natural Armor advancement, fewer bonus hit dice, fewer bennies all across the board.
May I also suggest looking at Dragon 341 as there are some articles on both wizard construct familiars and lesser golems. That might be a starting place from which to start building.
Also as templates go:
Particular: the clockwork variations:
Lesser Golems are definitely something I'll look into.
Where did the Clockwork stuff come from? Just out of curiosity and for future reference.
The linked pages actually have sources listed at the bottom of the page.
In the order listed they are:
Advanced Bestiary. Copyright 2004, Green Ronin Publishing, LLC. Author: Matthew Sernett.
Pathfinder Campaign Setting: The Inner Sea World Guide. © 2011, Paizo Publishing, LLC
Clockwork Creature Template from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc
I didn't think of Soul Binding. That solves the bulk of the issues with the creature. But, I still need to know if a Feat is in order which sounds to me it is.
The feat I linked to is simply one to boost the ranger's effective druid level for the companion, which he'd probably take anyway if he wants the companion to be better and not lag too fr behind that it's a hindrance instead of a help. Though the pathfinder version of the feat is better since it grants a +4 bonus instead of a +3.