Trying to connect Pegasus Raid 4TB to MacBook Pro MBP does not see the Raid. I also have an iMac 27" connected to the Raid with a thunderbolt cable, works fine.
You are attempting the impossible. You can't connect two Macs to the same Thunderbolt chain. The two Thunderbolt ports on the Pegasus are so that you can daisychain devices, not Macs.

Second, the Pegasus you have doesn't talk Ethernet.

You could use the second port to attach a display or another disk device but not a Mac.

Since your MacBook Pro doesn't have a Thunderbolt port you can't connect it to the Pegasus directly. You could use a Thunderbolt to FireWire cable, but even then not at the same time as it was connected to the iMac.