If another permanent enters as a copy of a creature with ravenous, it will not enter with any counters from the ravenous ability.

The triggered ability that checks to see if X is 5 or greater refers to the value of X that was chosen as the spell was cast, which may be different from the number of counters it entered with if there are replacement effects involved. This is also true for any other ability that it has which refers to X and triggers when it enters.

If Jacked Rabbit leaves the battlefield while its last ability is on the stack, use its power as it last existed on the battlefield to determine how many Rabbit tokens to create.

If a permanent spell with ravenous is copied, the copy will have the same value for X, and the token permanent that the spell becomes as it enters will enter with X counters.

A creature with ravenous gets its counters as it enters. It doesn't enter first and then get its counters. Any triggered ability that looks for a creature with a certain power or toughness entering will see the counters when it checks to see if it should trigger.