If you want to keg ferment, You can use a Ball Lock airlock. In my case, I sometimes ferment in PET Bottles. Notice the side mounted ball lock. I can rotate the bottle to point downwards. You can rotate the bottle in any direction. Keg fermenting is a great way to do pressure fermenting, using a spundit valve.
Using the bottle, gives you a visual indication of active fermentation, (co2 bubbling off) I use that as an indication of yeast lag and how long I get active fermentation.