I'm making a character with some racial levels and as such, she's getting a bit of a late entry to the druid class. So now I'm a little bit curious how the two following things interact if taken.
The Signature Mount pretty much gives a character an animal companion two levels below the character's hit dice if an appropriate skill rank is maxed out.
Meanwhile, a druid gets to choose between domains or an animal companion upon entry in the druidic class. In this case the companion would end up being four levels below character level, as the character has four racial levels.
Of course, the "boon companion" feat would be used in either case at some point.
So basically, what I'm wondering is that if the character takes the "signature mount" feat at third level, and goes into druid on her fifth, does she still get the option to get domains from the druid's nature bond or... how does the two interact?
It is its own animal companion. It uses the rules of druid companions but does not interact with any other companion you have. Thus when you become a druid, you can pick domains or a second animal companion. Or so I assume based on the wording. You could try contacting the company that published the 3rd party feat to see how they meant for it to interact.
It's a third-party feat, so you aren't going to get any kind of official answer here unless Tripod Machine responds.
For most abilities that grant animal companions, they stack only if both abilities grant the same type of animal companion. If not, then you get separate animal companions. For example, a cavalier's mount (horse) stacks with a druid's animal companion (horse) but not a falconer's animal companion (falcon).
That said, most GMs will nix any ability combination that results in you getting an animal companion with effective druid level greater than your character level--regardless of whether RAW supports it. This has already come up with the aasimar oracle favored class bonus and separately with Animal Ally.