Okay, let's have a go at the Runehound then. Ugly blighter, eh? I'd much rather be ambushed by Petals... Anyway, MM3 has it at CR3, so that shouldn't be too hard. Displacer Beasts, Hellhounds and Winter Wolves are all CR3 in 5e, so I've got some nice benchmarks.
Medium aberration, neutral evil
Armour Class 16 (natural armour)
Hit Points 45 (6d8 + 18)
Speed 50 ft.
STR DEX CON INT WIS CHA 20 (+5) 14 (+2) 17 (+3) 5 (-3) 12 (+1) 8 (-1)
Saving Throws Int -1, Cha +1
Skills Stealth +4, Survival +5
Senses blindsight 500ft. (blind beyond this radius), passive Perception 10
Languages Deep Speech
Challenge 3 (700 XP)
Pack Tactics. The runehound has advantage on an attack roll against a creature if at least one of the runehound's allies is within 5 feet of the creature and the ally isn't incapacitated.
Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) piercing damage plus 3 (1d6) acid damage.
Acid Stream (Recharge 5-6). The runehound exhales a line of acid that is 30 feet long and 5 feet wide. Each creature in that line must make a DC 13 Dexterity saving throw, taking 17 (5d6) acid damage on a failed save, or half as much damage on a successful one.
Glutinous Glob (Recharges 5-6). The runehound launches a glob of sticky goo at a creature it can see within 60 feet. The target must make a DC 13 Dexterity saving throw or be restrained for one minute or until it breaks free.
A creature restrained by the goo can use its action to make a DC 13 Strength check. If it succeeds, it is no longer restrained.