The creature that I converted on the fly was a Red Silt Horror from the Dark Sun Monstrous Compendium Appendix II (a really cool if lesser known monster book, illustrated all in color by Thomas Baxa who did, for the most part, a great job).
I beefed it up considerably to make it a challenge for my heavily armed PCs - the Dark Sun version has 8 HD and a 12' body, I made mine roughly 30 HD (180 HP) with a 30' body. I had it lurking, antlion style, in the sand.
What I have done when converting psionic abilities to mutations is to ignore the PSP cost of an ability - I usually just make it an at will ability, unless it is particularly powerful and the original monster only had enough PSPs to use it once, then I make it a daily power. Then I assign or roll a willpower score for the creature and use standard mental attack procedure from Mutant Future.
In this case, the Red Silt Horror used its abilities to lure the pilot of the hovercraft into its sand pit. This made for some interesting role playing as only the pilot of the hovercraft saw the incredibly inviting oasis, while everyone else saw the hovercraft slowing down and turning towards a sand pit. When Dataan wrestled Beyonce Chai out of the pilot's chair and attempted to drive away, he in turn experienced the mental abilities of the Silt Horror (Dataan has a pitifully low willpower). I had some fun with describing what exactly Beyonce and then Dataan saw out in the desert that made them want to stop the hovercraft and investigate - suffice it to say that I tailored it carefully to each player's desires, and in the case of Dataan and his player Carter, that made for some pretty hilarious moments.
In any case, soon the hovercraft was teetering on the edge of the sand pit and only the extremely quick thinking of Bozko the shape-shifting plant (riding behind the hovercraft in the party's jeep) saved it from slipping down straight into the gaping maw of the waiting desert behemoth. Bozko wrapped some tough fibrous roots around the jeep's tow hitch, made most of his body into a bow of sorts, strung with a vine, and made a small part of his body into an arrow attached with a thin fibrous cord to the rest of his body. He rolled an attack roll to shoot his body-arrow at the hovercraft, and as it flew he spooled more and more of his body into the trailing cord.
Last session marked the first appearance of Senor Carter's new d30, and I happily proclaimed us proud members of the Order of the d30. For the uninitiated, that means that each participant in the session (your humble Mutant Lord included) gets to substitute the d30 for any one die in any dice roll, once per session. Bozko grabbed the d30, and rolled something ridiculous like a 28 on his attack roll. I ruled that not only did he hit the hovercraft, he nailed its tow hitch attachment, so he turned his arrow into a loop of vine, grabbed the tow hitch, and poured the rest of his body mass into strengthening the vine-y rope that now linked the Jeep and the hovercraft. Logan, driving the jeep, put something on the gas pedal to keep it pulling the hovercraft out of the pit while he teleported into the fray that was just beginning.
The Silt Horror, realizing that its dinner was no longer sliding down towards its waiting gullet, erupted fully out of the sand and revealed its 8 writhing tentacles as it grappled at the hovercraft. Logan, arriving in the back of the hovercraft, grabbed his four-barreled missile launcher and ran to the front. Just as he opened the cockpit and put the missile launcher to his shoulder, the Silt Horror hit him with three tentacles and grabbed the hovercraft with 4 more. Undeterred, Logan took his last action as a living mutant - firing three missiles directly into the horrific beast (Logan has a mutation that allows him to take an extra attack a round, and also gained an extra attack from leveling up, so he gets to take a lot of attacks!). Two of the missiles hit, enveloping almost the entire hovercraft in the radius of the explosion (Logan may not have thoroughly thought this course of action through), instantly killing the already wounded Logan and two newly acquired slave boys (Dataan was quite upset) who were stationed at the forward guns and seriously messing up the exposed cab of the hovercraft and its other two occupants. Feeling generous, I ruled that the PCs still in the back of the hovercraft were unaffected.
Luckily, at this point the forward guns of the hovercraft were more or less directly pointing at the Silt Horror, so the badly burned and extremely ticked off Beyonce Chai and Sage (a mutant plant, a new character played by one of the original players who has missed a bunch of sessions recently) pushed the smoldering corpses of the slave boys out of the way, grabbed the guns and let the monstrosity have it. They both also rolled higher than a 20 on the d30, and I go with the old "a natural 20 is a double damage critical hit" house rule, so between the two of them and the damage the critter had taken from the rockets the Silt Horror was reduced to 3 HP. It used its escape tactic (using its air jet to creat a huge could of sand) and disappeared under the sands.
Whew, what an encounter! I should mention that the PCs chose not to follow the old road ways, but rather headed straight across the desert to save time, so I felt that they needed to run into something terrifying to remind them that cutting cross country across a Mutant Frickin' Desert is not without its perils. Danny (Logan's player) had a back-up character already rolled up, so I told the party that their fight had been witnessed by a stranger who came up and offered to guide them around "all the other Sand Monsters"! They gladly took him up on his offer, and we continued the session with the party's sense of fear and awe at the Mutant Future successfully restored!