Gillain holds her ground, 10ft away from the creature, between it and as many of the party as she can. She holds her shield and spear ready and takes the dodge action…

Tio, the closest to the creature as the horror unfolds, holds her action.

Ban: Ban lunges forward to strike the horse's body but, due to the burning and the creature's abrupt movement, she misses.

Gillain holds her action as well, placing herself as a barrier between herself and the rest of the party.

Gimble releases his spell at the creature. 
Save: 12
The flames rush over the reaching tentacles and the corpses of the horse and rider. The bodies crackle and pop as the smell of burnt flesh reaches your noses. The tentacles catch flame and quickly withdraw back into the body.

 DM; will wait for Ban's action. 

Ban takes her silvered blade and lunges forward to stab the would be horse then hops back with a Disengage.

Attack: 11 Damage: 10

The creature seems to have withdrawn into the horse's body due to Gimble's attack. 
The rear of the horse seems to shudder for a moment and then movement stops.

DM: 
Bob can act now. 

Bob takes advantage of the tentacles retreating to cast Entangle  on the horse-creature-thing, vines growing below it which also span twenty feet in the direction of the party so if it passes its strength check there's still difficult terrain for it move over.

Bob cast his spell.
Erupting from the ground around the horse carcass is a series of vines. Not green lush vines that would normally be created by the spell but old greenish-gray vines, full of thorns and coarse bark erupt around the bodies and entangle it. 

However, since the party had posted they had moved in close proximity of the beast,
Gillain has stated within 10 ft. (post #1665)
Ban got roughly 10 ft. away from the creature (post # 1645)
Gimble stood his ground, within 15 ft. of the horse (post #1648)
And
Tio walked over to the horse (post #1653)
I will need everyone to make an Str saving throw or be entangled by the vines. The DC is 14.

Ismark and Ireena had moved closer as well...moving with the party so:
Ismark Save: 12
Ireena save: 12

Both Ismark and Ireena failed their saving throws and the vines entangle them.

STR save: 13

STR save: 10

Gillain’s strength save: 11 

Gimble:

Gimble backed away after casting his spell (post 1664), though he did not specify how far...

Strength save if needed:  6

The vines grow rapidly, their rough coarse tendrils entangling the entire party, save for Bob.
After the shock of what has happened has worn off, everyone pauses, waiting to see if something erupts from the horse's corpse.
Time passes so slowly it is hard to believe your pauses are so brief. You wait with bated breath for something to happen and yet nothing does. 
No movement or sounds come from where the corpses lay. 
In the distance, the howl of a lone wolf can be heard. 

Actions?

Caught by the vines Tio gives Bob a stern look through her mask before using her bonus action to summon her astral form. As the ki energy burst the horse takes 8 force damage. Then as her action she will use her arms to break the vines! 

Str(Wis) Check: 21 

[With the astral arms I can substitute wisdom for STR check/Saves.]

Ban struggles against the thorny vines, trying her best to wiggle free without catching herself on the thorns but ultimately is unable to do so.

Str check: 6

Gillain remains calm. She appreciates the vines, keeping everyone in place and safe and apart from the abomination before them. She holds her spear out towards the creature, then flexes a little to try and free her arms and legs…

Strength check: 7 

Gimble:

At the outer edges of the entangle zone, Gimble tries to catch a glimpse of those tentacles, not content to attack what seem to be the host corpses as that may not actually damage the creature.  In the meantime, he tries to figure out what kind of strange creature this could be.

Intelligence Check:  12

(Readying Magic Missile if he sees the tentacles) 

Tio takes a couple of deep breathes the reach calm within herself and then summons her astral form. Two translucent arms appear around hers and she strikes at the horse's corpse. Her aim is true and she hears ribs crack and the flesh concaves at the area she strikes.
There is no response.
With the help of her astral form, Tio is able to break the vines that entangle her. 

Both Ban and Gillain struggle to break the vines but, are unable to do so.

Gimble shifts as best he can to get a clear shot. He takes a second to think about the tentacles and what creature they might belong to but, nothing comes to mind at the present.

Bob?

After a few short moments, the spell fades and the vines retract back into the ground from where they came.
"Is it still there?" you hear Ireena ask in a fearful voice.

DM:
Tio-yes, I will allow you to substitute that check. 
Gimble: to pin down an idea about the creature roll an Arcane roll.

Everyone consider yourselves out of combat.
Actions/interactions?

Gimble:

Arcana (with proficiency) makes that a 16.

As Ban is released, she briefly rubs the contact points and looks around at the group. Uneasily moving to the side, she says "Let's just go, alright. The quicker we are to Vallaki, the better. Fairing a night out here isn't looking to be rather fun, with the wolves behind us and whatever the **** that was." 

Gillain notices Gimble is thinking hard about something. “Penny for your thoughts on all this, Gimble… … What troubles you? Have you noticed something important?…”.

As she asks, Gillain keeps her spear pointed at the quietening heap of horse bits and tentacles.

She also nods at Ban’s suggestion to move on, and starts making moves away from this creature, encouraging everyone onward, whilst all the time trying to keep herself between the ‘thing’ and the rest of the party.

She wouldn’t turn her back on it until everyone was a good 100ft or so away, and even then she would keep one eye peering back over her shoulder every so often.

Bob mumbles a sheepish "Sorry, folks," before shuffling along, holding out his gnarled walking stick as if to keep the corpse-pile-thing at bay.