This one's so great for spinning out overconfident parties.
It just uses the fire salamander stat block with slight changes. [see below. ]

Trick is, when it gets to half hp (200->100) it splits into two of the same monster, with 50hp each. Then if either of those gets taken down to 20hp, it also spits into 2 more with 10hp each.

Changing the 'bite' to a 'strike' is the only change you have to make besides adding that the cold vulnerability also prevents the damage from causing a split.

This lobster-like abyssal minion hides in the midst of regular lobsters caught in traps.

They lie in wait, only to dominate the mind of any who touch them, using them as thralls to hunt future prey.