Well, let's see here. An 8/8 for 8 with trample. That's not very good, especially considering that you have to pay Eight green specifically. But wait! There's more! For every green creature you control, he's gonna cost you one less green mana. Now that's very promising.
The best use is not so much to actually swinging with him, but to catch the attention of your opponents removal and/or kill. Very few people can let such a powerful creature go unchecked.
Strategies and game play:
I'd recommend using Khalni Hydra in a deck with several weaker, cheaper creatures. He's an invaluable add-in with an elf deck or mana ramp deck. Elves are generally one or two cost creatures, and in with Llanowar Elves, Arbor Elves, and Birds of Paradise they also ramp up that mana a little bit.
Combos with other cards:
This card really shines on it's own. However, it does work great with pump cards like Primal Bellow and Groundswell since it's a natural trampler. It's also a good deal of fun to throw Canopy Cover on him, or equip him with a Whispersilk Cloak. And in case of Doomblade, Vines of Vastwood.
Ways to counteract it:
Oh boy... now here comes the sad part. This guy will instantly catch the attention of you're opponents removal, counters, or kill. Unless you give him shroud, he's going to be the subject of just about anything bad. Pacifism, Condemn, Journey to Nowhere for white. Cancel, Mana Leak, Mind Control for blue. Black can always Doomblade or Go for the Throat, or worse, Gatekeeper of Malkir. Red will have a little trouble, but can always stack it with their burn.
It's big, is has trample, and it can be cast as soon as turn three or four.
It doesn't have too much evasion, and it can be difficult to hard cast if you got screwed out of cheap, green creatures.
It was introduced in Eldrazi, so there is only the regular versions.
Artwork and aesthetics:
Eight vicious gnashing hydra heads leading back to a massive tail. I don't like the whole thing having two legs, but I guess that's kind of the classic hydra look. And as is always my favorite with big creatures, people at it's feet for size comparison.
I'd say this card is about 7-8 out of 10. It's very easy to get rid of, but he's also easy to cast and draw away a bunch of kill from your other creatures.