It will usually be very difficult to separate one from the pack. I suggest trapping all of them (usually 3 in a pack) in a behemoth gate or ramp style trap and kill the ones you dont want before taming the one you do want.
Is it just me or is the allosaurus broken on mobile Every time I leave my base my allos teleport outside my base and end up in random places near my base another thing is they don’t want to lay eggs but I have 2 males and 2 females and a oviaraptor
Try to make them fall into a ravine. Flyers help with this tactic. Or better, look for some that spawned or fell into one. There’s a ravine on my SP game where there are around 11 allos that fell into it. After getting them into a ravine, kill off all but the one you’d like to tame. Proceed in taming as usual.