D&D 5E Summons and Rolling Initiative

[Fanaelialae]

Another option is to roll the initiative of the creature and just have it act on the caster's turn for that round and then use its own initiative for rounds after that.

So, if your caster goes on 10 and summons a dung beetle and the dung beetle rolls a 20 for its initiative, the dung beetle will act as soon as the caster summons it and then will act on its own initiative in the combat order thereafter (so, it'll then act on 20 in the rounds to follow, in this example.)

That seems potentially a little too good. It could result in the summon(s) getting two turns in a row (for example, if the summoner is last in the initiative order and the summon(s) roll first in the order). Given that most of the summoning spells are already top notch, it seems like this could push them over the top. Admittedly, it's unlikely to happen every time you use a summon spell, but it will almost certainly make an encounter where it does happen significantly easier than it would otherwise be.

 

[CM]

One thing I've been toying with is setting their initiative at 2d8 lower than the caster, modified by their own initiative bonus.

 

[MechaTarrasque]

You could make it so the summoned critter acts after the summoner on the first round, but the summoner doesn't have time to give it any commands--in effect it just shows up and if something attacks it that round, it fights back. I think this fits that fact that most of the in-combat summoning is for things like a bunch of animals, sprites, or mephitis; it will take a bit just to see where they all ended up (not to mention what you actually got from the spell). I think it would be pretty rare that you would just happen to finish one of the longer summoning spells just as combat began.

 

[Grakarg]

I prefer to keep the rule as listed in the PHB, having summoned creatures (and familiars, and pets, and anyone else) roll their separate initiative.
While it does slow things down a little to add an extra turn into the order, keep in mind that Beastmaster Rangers have the special ability for their animal companion to act on the same turn as the ranger.
So if you allow this for everyone, you actually give everyone something for free that Rangers get special, which I think that takes something special away from them. Rangers have enough problems as it is, let them have their coordinated initiative attacks.

Its actually pretty powerful to coordinate several attacks all on the same turn with your characters, and requiring separate initiatives helps smooth it out a little.

 

[Draegn]

In my game, if the summons is successful the creature waits to be given an order or task. If the summons fails and the creature still appears it is free to do whatever it wishes to do.