Would this be acceptable counts as?

[Lord Kallozar]

Is there any way or possibility that a Chaos terminator could used as a counts as daemon prince? Obviously the height is a factor but is there a way around it? What if the terminator lord was heavily mutated would that be enough to count it as a daemon prince?

[Panzer]

Depends on your opponent but I'd say it'll be more of a proxy than a counts-as so not allowed on most tournaments and more like tolerated at best for others. After all a Terminator is an existing unit in the game and looks and acts very differently than a Daemon Prince.

 

If you'd do extensive conversion work on him it's more likely to be tolerated as a proper counts-as.

[Azekai]

A 'regular' terminator lord obviously isn't a daemon prince, but feel free to convert your own freakish alternative. As long as it is a similar size to the DP and looks suitably monstrous, you should be all set.

30k primarch models also seem like decent stand in candidates- or even something like one of the Iron Circle. Stand-in DPs should be big, imposing, and not easily mistaken for anything other than a giant monster.

[N1SB]

I agree with the above, but there was one interesting combination of Daemon Prince with a Legion Artefact where Terminator armour made sense IMHO.

 

It'd be an Iron Warriors Daemon Prince with Fleshmetal Skeleton.  That Artefact gives the character 2+ and, combined with his Daemonic save of 5++, it'd be kind of like having (Tartaros/40k) Terminator Armour.  I thought it'd be a fun yet intuitive way to represent the 2+ save.

 

Obviously the height is a factor but is there a way around it? What if the terminator lord was heavily mutated would that be enough to count it as a daemon prince?

 

And the biggest reason for using a converted Perturabo model was precisely because I didn't want to seem to be "modeling for advantage", the height issue.

[Sonoftherubric21]

General rule of thumb when using a "counts as" equivalent is you want something of "about" the same dimensions give or take; 

as stated above the iron circle used as Iron Warriors daemon princes is a great example; the Morghasts from age of sigmar as daemon princes with wings are famous counts as conversions, you need something "in the size range" 

As a personal example; 

 

https://4.bp.blogspot.com/-IIEFHA0ygyE/WJz-_4Bg17I/AAAAAAAACP8/psiKU8EETHwpPduWSdzOL7KiuLiaTp-cgCLcB/s1600/thousand%2Bsons%2Breview%2Brobots.JPG

I use those as a "counts as" for a thousand sons helbrute, they are on the same base size, are about the same width, and only a hair taller then regular helbrutes, never met anyone who says no to me using them. "rule of cool", fluff, and most of all reasonable assumptions, the more obvious it is for your opponent, the better off you are with it on the table.