In the past they were only available via the Creature Illusionspell, but after they were introduced to the game they were removed from that spell. It is the only creature that was added in the Christmas Update 2005.
The Library on Tibia.com describes them as follows:
"The hideous Serpent Spawn is rumoured to be the creation of some ancient serpent god once worshipped by the lizardmen of Tiquanda. In ancient times, this creature, chosen by the gods, was a vital part of the lizard culture and the centre of numerous gruesome rituals. With the downfall of the lizard civilisation, the Serpent Spawn became more and more uncontrollable, and finally started to become a threat to the lizards themselves. As if their minds had been darkened by a madness that mirrored the turmoil in the lizard society, they made no difference between friend or foe and their lethal wrath struck both. By making the most holy places of the lizards inaccessible, they sealed the fate of the lizard civilisation during the apes' revolt. It is rumoured there are still species of the Serpent Spawn around. Still guarding the sacred places with eternal vigilance, they are waiting for their creator to rise again and to lead them once more to reign the jungle of Tiquanda on the god's behalf. Being servants of the serpent god, they are blessed with magic abilities and their serpent heritage grants them enormous physical strength and constitution."
They hit hard with their waves, so try to keep diagonal to avoid Earth Wave and Musical Notes, a good idea is to bring Garlic Necklace, since Musical Notes do Life Drain damage. Blockers should use a firebomb or energybomb while shooters stay away of its beams, as the creature retargets very often and runs on low health, try to finish them quickly when they start running, as following them into a group of Serpent Spawns could be lethal.
Paladins: A level 150+ Paladin should have no problem soloing these. Just use Assassin Stars and Divine Missile combined with Light Healing and Divine Healing if your hp runs too low. It can be hard to avoid the waves by running, especially in the narrow passages of some of the places where they reside so it may be easier to just sit there and take the melee hits so you don't get waved. They run in low hp, but this isn't really a problem for Paladins. There will usually be profit in loot.
Knights: If you're a level 150+ Knight, you should be able to solo these quite comfortably. Always stay diagonal so you don't take their waves, then just combine your melee with Whirlwind Throw to take them down in reasonable time, heal with Wound Cleansing and if you get really low, heal with Ultimate Health Potions. Be careful when it runs, try to get it to run in a direction from which you've already cleared all the monsters so you don't get swarmed with them. If there's 2 at once, use Berserk to try to get rid of one as fast as possible while still damaging the other one. If you're a really high level, 200+, a good strategy is to find a Druid to heal you, gather 3-4 at once and use Fierce Berserk. Great experience can be gained this way. If you just use Whirlwind Throw, you should profit in loot. If you are using Fierce Berserk, you will probably break even.