Good Beasts to Capture

Giant fucking Hoarder spiders, and Umberdroths.

CAVE BEAR !!!

Really big rock spiders !! (they spit acid and tank for you)

I like corpse spiders, also skaths and slags. I hope you can capture demons and dragons but I have not tested it that far yet.

My first priority is for spiders that can web (Black Widows, Corpse Spiders, Harridans) because they really help reduce the risks of trying to capture other critters.

It is very important to get your Spell Mastery as high as you can get it, and given how expensive Tame is at the start, it helps to get promotions or equipment that also helps with the mana cost of tactical spells (Mana of Oceans really, really helps, but it requires level two in Life and Water magic).

Harridan and Hoarders spiders and Umberdroths are probably the best for tanking ... Cave Bears aren't too terrible ... and Great Wolves are always good to have along in any fight, but be careful as GWs are not themselves great at tanking (though self +Enraged with howl, they can get some decent strikes in).  Also remember, the +Enraged attack bonus that the GW's howl increases with the GW's level, so its important to try to capture GWs early on (but you may have to wait for wolf packs to grow/level up into a Great Wolf) and bring them with you through many fights to level up.

Stalkers (Plague or normal) are the fastest tactical units I know of and are really good for getting around enemy melee units entirely on the battlefield to take out their archers and ranged spellcasters in the rear.

Silt Skaths aren't to be overlooked, either ... no, not good tanks, not a huge amount of hit points compared with a Giant Harridan or Hoarder spider, but decent movement, decent attack that poisons and a special attack that does triple the normal damage, doesn't trigger a counterattack, and imagine a Great Wolf-boosted attack that is, then, tripled.

Umm, the game won't let me try to capture a slag.

You cannot capture demons nor dragons.

It is very helpful to have or bring along a spellcaster who can cast Haste if you want to capture more than one critter in one tactical battle, as the cooldown for Tame is 10 turns ... that's a big reason why webcasters are helpful, even beyond the first one you capture, to help keep the other critters you want to tame from melee attacking.

Is it because they resist or is it due to being an invalid target?

Invalid target

Cave Bear is the best I have captured.

interested in trying a drake or a slag.

interested in watching you try. Send the screenshot.

Dragons, Drakes and Slags are invalid targets, the game won't let you cast Tame on them (at least not the spell, not sure if the collar would be any different).

Giant Harridans and other spiders are probably the toughest beasts you can tame.  Umberdroth is probably the most dangerous overall, with an initiative of 28!

The monster has to be type "Beast" to be capturable.  Dragons and Drakes are type "Dragon" and maybe Slags are as well.

I would rate silt skaths and umberdroths as equally useful.  The former has the deadly bite, which does not miss and does not provoke a riposte.  It's more situational than the umberdroth tanking ability and steady damage, but front-loading is useful.

The hoarder spiders make the best tanks, but they become obsolete too.

The great wolf is the one that remains valuable at any time of the game, even when you are fielding high end armies and battling full stacks.  Its Howl ability cannot be overestimated.

I captured a Harridan Spider with a collar, and decided to grow it. I casted tutelage, so it has arrived to level 13 with ~140 HP. Not bad, but as Tuidjy has stated, at this point it seems a bit out of the endgame, as the defense remains in only 5. Anyway, its web skill and the tanking has been very usefull along the early and midgame.

Anyway, it gives sense to the beast lord trait, and I think I want to play it on a next game, and get some pets. I think that combining it with the "free demons from shards" (binding?) might make a cheap and easy way to troop in the early game.

Updated initial post.  More ideas?

The collar works the same way as the spell, so it wouldn't be any different. 

I have gotten the most mileage out of the various spiders (crowd control).

Many beasts are too frail to keep them around for long, though. I liked the cat-like beast which had such a high movement and a poisonbite,  that could not be counter-attacked. But durability was on the low side.

Slags definitely don't work, I have tried.

There are also these snakes having jewels in their body (forgot the name).

They have a special attack (poison?), are they useful as a beats? They have 4 moves and about 120 hp, making them agile while having a bit more health then the skath?

Don't know, haven't tried them..

Plague stalkers.  There's a non-plague versions of those cats, too, just plain stalkers.  Yeah, I agree, as I said in my post, their high movement points make them excellent for going around the front line units of an enemy and attacking the archers and casters they want to protect in their rear.

I would have but the information below leaves that point mute.

Elementals

Beasts

Etc.

Edit  Does the collar work exactly the same as the tame spell for Beast Lord

Great wolf is the only thing I would use a collar on (I find beastmaster too op, so I don't play as one). It starts with giving +6 atk to all units... and by the end is easily giving +15-20 atk to all units (making stacks of units 1-hit EVERYTHING). By far the most powerful tame in the game. It will turn a bunch of worthless slaves into dragon slayers. Also has the highest initiative of the bunch (32 if I recall correctly). I usually rename him to "Dues Ex Alpha". Will break the game if you use it... and might need a nerf (say: +5 atk instead of +1 per level).

My favorites are the corpse spiders, Naja's and the other snakes.

Hense the ....was teas'n a bit there.

You find the creature type by doubleclicking on it on the strategic map, and seeing its type.  Basically it is the screen that you can get for any unit in the game - if it is a "Beast" type, you have a chance of capturing it.

I believe you are referring to Ophidians.

Speaking of, I'll also throw a vote to Ophidians, and also to any spiders with the ability to web enemies, and Great Wolves, if you can survive the lupine army.