The Abyssal Spawns (Rhino, Horse and Demon) seem to either de-spawn or start attacking you after a while.
The timer is soo short for them they are next to useless, I assume there is a server side setting to lengthen that however as standard it is very short if it is intentional. I would imagine that the Abyssal spawns would be 1. Limited in number , i.e. one of each and 2. last as long as you are logged into the server.
I had an Abyssal Mount (Rhino) out where I was collecting ore and coal in it and it despawned before I got it back to base loosing all materials it had in it’s inventory. The demon I had summoned with Abyssal Call stopped following me and started attacking me at one point as well as you can imagine that ended badly as well and I lost everything it was carrying.
There is also no indication to what these things like to eat or heal themselves with so far I’ve worked out that Human flesh is the food of choice but it would be good if they had a bit of info on them to let us know.