Did butterfly drills last night and noticed how tired my legs got.
Aside from continuing to practice my butterfly are there any particular exercises that would strengthen my legs?
I've found that really only helps my closed guard.
Hang from the "Ab Straps" sling aparatus, keep your knees bent and curl legs as high as you can to your body - and do some boot strappe squats afterwards. Tranquillo?
I'm not sure if the strength when using butterfly comes so much from your legs as it does from getting your hips under you legs.
asscobra - I'm not sure if the strength when using butterfly comes so much from your legs as it does from getting your hips under you legs.+1
But you can also strengthen the muscles around the movement. Try strapping on ankle weights and mimicking the movements for sweeps and posture movements. The legs are really just "extenders", though. Mr. Asscobra is correct.
Leg extension machine. My butterfly guard wasn't too great until I incorporated that exercise into my routine. Either that or just continue to work butterfly guard and your legs will naturally get stronger 
butterfly guard doesn't really require strength as opposed to proper mechanics. just drill and spar more from that position.
I feel I use more core strength than leg strength from butterfly. If you find yourself trying to lift with solely your legs, you may want to reexamine your positioning. I find that when you're under your opponent's base and have him off-balance, the sweep requires much less power.
A great drill for this is having a partner get in your butterfly and pull him on top of you. As you pull him roll on to your back (not good technique for the actual sweep, but for the drill) and do a leg extension, basically lifting him in the air and allowing him to base. Keep repeating for a minute at a time. This will work on your leg strength and core strength for the position.
I play mostly butterfly and don't understand this ... it doesn't take any leg strength. If your legs are getting tired, you are doing it wrong, probably trying to "kick" your opponent over you with your legs.
Your legs do almost no lifting. At most, they hook, but they should be bent, thus requiring no strength to hook.