Toad Dragons are a breed of monstrous reptile sometimes allied with the forces of Chaos, usually Nurgle, in the world of Warhammer Fantasy Battle. They are a Forge World creation, released as part of Tamurkhan: Throne of Chaos; Tamurkhan himself rides a Toad Dragon by the name of Bubebolos, whilst the book also contains stats for including Toad Dragons as monsters via the Storm of Magic rules.
Toad Dragons are enormous, reeking, primeval horrors from the Cold Mires of the Chaos Wastes, earning their name for their squamous, reptilian yet toadish forms and immense power. Despite this, it seems they are more likely some breed of primordial lizard, akin to the saurian beasts that infest Lustria and which are tamed by the Lizardmen, rather than some profane branch of the Dragon family tree. Near-mindless, endlessly hungry and unthinkingly violent, Toad Dragons are colossal creatures that are almost impossible to kill, with strength as near-endless as their appetite. That which they do not rend apart in their gaping maws or tear apart with their claws is crushed under their bulk, struck down by their flickering, venom-seeping tongue, swallowed whole, or dissolved into rancid goo by their unspeakable breath weapon, a corrosive gas that liquiefies flesh and steel and leaves nothing but gore-flecked pus in its wake.
Use the Bound Monster rules from Storm of Magic, Toad Dragons cost 350 points for a single-model sized Monster typed unit. Is it worth it? Well... for starters, it's a hell of a lot faster than you'd think (Movement 8!), and virtually impossible to kill (Toughness 7 - plus the Colossal Beast rule below - and Wounds 10!) whilst hitting like a dumptruck (Strength 7). On the other hand... its combat stats aren't that great (Weapon Skill 4, Initiative 2, Attacks 4, Leadership 6).
In terms of special rules, Toad Dragons have the Universal Rules Large Target, Scaly Skin (3+), Immune to Psychology and Terror, as well as the unique rules Colossal Beast, Unspeakable Foulness, and Tongue Lash.
Colossal Beast makes a Toad Dragon almost indestructible. For starters, they can't be hurt by anything that isn't at least Strength 4. Period. And no matter how strong or what special rules the attacker has, Toad Dragons can't be hurt on anything better than a 3+. Attacks or spells that'd cause Instant Death? They just inflict D6 wounds instead, thanks to this rule. As a side bonus, Toad Dragons deal out 2D6 hits when they use Thunderstomp.
Unspeakable Foulness is the Toad Dragon's Breath Weapon, which requires those struck to take a Toughness test at -1 or suffer D3 armor-ignoring wounds.
Tongue Lash gives the Toad Dragon an extra attack that is Strength 4, Poisoned, and Always Strikes First, which can be dealt against any enemy model in base contact. Those hit by this attack and surviving still take a -1 penalty to their To Hit rolls for the rest of the combat turn.
All in all, Toad Dragons are expensive, but could potentially be worth it. Thanks to their huge array of wounds, their insanely fast movement and Colossal Beast, these things are the ultimate anti-tarpit unit; able to charge 16 inches, a Toad Dragon can slam into almost any tarpit unit first. It may settle to a war of attrition, but as long as the tarpit is tied down with the Toad Dragon, it's not bogging down any of your other gribbly units, and with 2D6 Strength 7 Thunderstomp hits per turn, well... most tarpit units won't hold out for long.
In fact, Toad Dragons make pretty damn good tarpits themselves. Throw one of these things at a big nasty killy monster or character, and between its 3+ save, Colossal Beast and 10 Wounds, they are not getting away from ol' toady any time soon.
Bubebolos is an upgraded Toad Dragon that can only be taken with Tamurkhan, it essentially functions like an even harder to kill Toad Dragon since it gains Mark of Nurgle and +1 WS (meaning everyone WS 5 and under targetted by his Tongue Lash can only hit him on a 6), along with +1 Strength, +1 Ld and an upgrade to his Colossal Beast rule, making it so that he can only be hurt on a 4+. Greatest of the Toad Dragons indeed.