Sweet, though this acts more like a "mutant" than "toxic" critter I think (and note, all this is just GM 'criticism' in the academic-analysis kind of criticism, not in any way mean criticism).
For toxic paracritters (not toxic normal critters), just two toxic powers are added (attributes, skills, etc. stay the same).
Mutant critters, on the other hand, can have attributes modified (up to x1.5 base attribute) but for every 2 points the critter loses 1 Essence, so the Cocodrilo del Infierno would have a max Body of 15 (1.5 x Behemoth's natural 10) and would loose 2 Essence (and also -1 Essence for +2 Agility, -1 Essence for +2 Reaction [max Reaction of 5], -1 Essence for +3 Strength, -1 Essence for +2 Intuition, -1 Essence for +3 Willpower [max Willpower of 8] and then, because of the total Essence loss being 7 points [more than it could handle anyway] for every 2 Essence loss, -1 Intuition, so -3 Intuition). Adding or increasing skills and adding qualities also reduce Essence for mutant critters.
As written, it's more akin to a new species, a genetic cousin to the Behemoth, than a toxic critter and in order to keep it as close to how it's written up I would make the following changes:
1. Drop the toxic aspect, and just make it a new species.
2. Change Range to be: "As a cousin to the Behemoth, the Cocodrilo is also found in the subtropical portions of North America's southeastern region but reports have placed individuals as far north as Missouri on the banks of the Mississippi." (or something similar).
3. Frequency: Rare or Very Rare. (an "Unknown" frequency means no one has ever been sure of its existence, nothing has been confirmed. If 1 and only 1 has been confirmed then it's Unique, 2 or more gets into the Rare range.)
4. Increase its size to 4m at the shoulder, as the Behemoth stands 3m and has a Body of 10. Either that or lower it's Body to 8 - 10.
5. Remove the Qualities line (unless you give it Positive or Negative qualities, Essence Drain is a Power).
6. Change Powers to: Essence Drain (I have a hard time with this power, given the nature of essence drain I don't see a justification for having it...at least a giant alligator subduing and terrorizing it's victims for minutes at a time in order to drain their essence...why?), Hardened Armor (6, I lowered the rating from that of the Behemoth so that it could have a "toxic" feel even though it's not a true toxic critter, the softer armor can be described as appearing sickly and soft compared to that of the Behemoth's Hardened Armor rating of 10), Natural Weapon (Bite: DV [Behemoth is 14P, so if you keep its size at 2.5m I say drop it to 10P or 12P, and if you increase its size with its Body, then up it to 15P or 16P], AP -2), Pestilence.
7. Skills: Add, Infiltration 3, Unarmed Combat 6. Drop, Shadowing
8. Reduce Reaction to 4 (Faster than a Behemoth, but not making it wholly unlike alligators)
9. Reduce Willpower to 6 or maybe 7.
10. Increase Magic to 4.
