An NPC, naturally, requires both code and assets.
Assets are already compatible so you just copy the models and the textures.
The code is mostly compatible but needs to be edited a bit, in key spots.
If you're a complete newbie to coding, read this
https://developer.valvesoftware.com/wiki/Getting_Started_On_Source_SDK_2013And get Visual Studio 2015 Community, it's the best, in my optinion. Although I haven't tried 2017 yet.
Now that I looked in the code, it seems that 2006 SDK version of the houndeye is incompatible because it misses a few files, they're simply not present.
So we'll go with HL:S version.
When you copy the code of hl1_npc_houndeye.cpp and .h into your project (into Server), you'll immediately notice a few red flares, erros in the code. The first one is that the NPC wants to include the file hl1_ai_basenpc.h, and you could copy it, but it's far easier and less messy to just change it to "ai_basenpc.h", omitting 'hl1', and using HL2's basic ai functions for navigation and such.
Then you'll see that in the header file, hl1_npc_houndeye.h, the creature is declared as a CHL1BaseNPC. We should change to CAI_BaseNPC (because CHL1BaseNPC is defined in hl1_ai_basenpc.h, and we already decided not to use it).
Then let's look at the cpp file. It also has a few flaring error.s
One is that the creature is classified as CLASS_ALIEN_MONSTER. This relationship class (used for determening how one type of creatures percieves others, for example CLASS_COMBINE always hates CLASS_ZOMBIE, or CLASS_ANTLION hates and attacks most other things) is not present in HL2. You could add this class in hl2_gamerules.cpp, or you could change it to use one of the existing ones, for example CLASS_ANTLION.
That'll make the houndeye hostile to the player, to Combine soldiers, player allies, zombies and whatnot, and naturally, it'll make it neutral towards anltions (since they'll be sharing the same class). If you're not planning on mixing the two in one area, it's fine.
CLASS_ALIEN_MONSTER is referenced a couple times in the cpp, so you should change all instances of it to CLASS_ANTLION or whatever you chose. Again, the compiler lights up all errors in red, so you can quickly find them.
In case of the houndeye specifically, it uses some fancy beam magic to create shockwaves, and it absolutely needs the sprite material, sprite/shockwave.vmt, to be present, to be copied from HLS to your mod, or it'll crash. The path to the texture is declared inside the Precache() function.
Anyway, here I include the houndeye files which should go in your mod's sourcecode Server folder, or Server\yourmod for better file organising. I fixed everything I wrote about above, and tested it in-game.
https://drive.google.com/file/d/0B62rJ8GDQ_WnelRNUTRyTTlXWnc/view?usp=sharinghttps://drive.google.com/file/d/0B62rJ8GDQ_WnWGlfaEYzNGFuYlk/view?usp=sharing

You should also remember to define health and attack strength in your skill.cfg, the commands would be sk_houndeye_health and sk_houndeye_dmg_blast. It will work if you don't define them, but you wouldn't be able to tweak them w/o editing source code every time.
And in case of the bullsquid, it's very similar, you also need to fix the includes, fix the classify function to return something other than CLASS_BULLSQUID, and I think you need to change the spit grenade calls because they're slightly different in Ep2 as they were changed for the antlions, so they take different arguments for spawning, or at least I think so from what I remember.