So my game converted recently from pathfinder. Our cleric was completely built around the idea of animate dead and was having a blast animating the crazy things we killed. Once we converted to 5e though, animate dead had a significant nerf, changing to only being able to animate humanoid zombies and skeletons. This has caused the player to become a bit disenchanted with his character since the best part of it was his zombie monsters. 
Does anyone have a method of converting creatures to zombies to be used for Animate dead that doesnt completely break the game so that he can relive the days of his skeleton direwolves and zombie Behir? Open to any suggestions lol

If I were to try to make this work, I'd do a sort of re-skin of Conjure Animals, eliminate the "beast" requirement, and work with the player to find a happy medium where casting the spell with a higher spell slot would grant access to higher CR monsters. Then, have everything he conjure just count as undead but generally use their normal stat blocks. 

homebrew a higher level spell

For the 5e summoning spells, its generally a creature of CR equal to or less than the spell slot, and you get it for an hour with concentration. So I would say just let him higher level cast (if he has the appropriate slots available) and he gets an undead version of the creature for an hour with concentration. If they are lower CR (2 or less) just follow how Conjure Animals works.

If he wants it for longer / without concentration, first, I would only let him have one raise effect active at a time. Second, I would limit the creatures. A -1 to all rolls / saves and reduced HP (half) is probably a good starting point.

A behir in 5e is CR11. I don't think anything short of Miracle could raise it without it being game breaking.

I agree a good bit with Houligan, but I add in a little bit of the research I did on the stats of the conjure spells.

Conjure spells usually all lets you summon a creature of CR equal to half the spell level used (rounded up, so 3rd spell level would still summon a CR 2 creature)

They all last for 1 hour and require concentration. If concentration is broken then the spell seems to end which causes the summoned creature to disappear.

Exceptions to this:

Conjure Fey Creatures - makes it an even 1 to 1 for CR and Spell Level. So a Spell of Level 9 summons a CR 9 creature. The penalty to this is, if you lose concentration, the creature does NOT disappear. Instead it becomes hostile to your and your companions.

Animate Dead - Lasts for 24 hours instead of 1 hour and does NOT require concentration. Its penalty is that it only raises two basic types of undead and they are CR 1/8th.

Summon Greater Demon - You get to summon a demon of a CR that is +1 to the Spell Level. Its penalty is that the demon makes a saving throw at the end of each of its turns to break free of your control. And if you stop concentrating on it then it remains and is free of your control for a few rounds before disappearing.

So there would be an idea of how to tweak a spell or homebrew one that would easily exist in the rulebooks and give a necromancer more options. Maybe there is also a standard "undead" template, but I haven't checked in the listed creatures or rulebooks for something like that.

I thought there was a zombie template, but I guess not. I made a rough list of common traits that you can use like a template.

Speed -10, -2 DEX, 3 INT, 6-9 WIS, 5 CHA, poison(ed) immunity, darkvision 60, undead fortitude

INT, WIS, and CHA are never increased from original. WIS is usually 6~7 less than original (modifier reduced by 3). High DEX creatures sometimes have -4 DEX instead. Monster's other abilities may be reduced by rotting (ie beholder zombie).

Okay, so I compared about 7 of the unintelligent undead that I had access to to their living counterparts and I tried to spot things that were common (there are usually some exceptions.)

Here's what I found: 

Alignment - Zombie Undead are usually Neutral Evil and Skeletons are usually Lawful Evil

Speed - Zombies usually get -10 ft if they have a normal movement speed.

Hit Dice - The same type of Hit Die is used to calculate health (d8, d10, d12, etc) but the number of dice used changes (low level the Hit Dice number goes up, high level it goes down)

STATS:

Str - Usually stays the same

Dex - Zombies get -2 to -6 Dex

Con - Boost to Con (+1 to +6)

Int - Low Int (2 to 6, usually a 3)

Wis - Skeletons usually have 8, Zombies usually have 6

Cha - Almost always 5 across the board for these unintelligent undead

Challenge Rating - Except at the lowest levels, undead have a lower CR than their living counterparts (beholder zombie is WAY lower though.)

Pretty much the same template I came up with. You forgot darkvision and poison immunity though, and zombies all have undead fortitude.

Darn! I wrote down in my notes that all undead have 60 ft of darkvision (even if their living bodies had greater) and the poison immunity but I forgot to put it in my post. Didn't notice the undead fortitude though.

Maybe I'll research ghost type undead later.