Can I play Grim Apothecary, trigger effect on itself, react with Rengar Legend effect on another unit and take back to hand the Apothecary?

Yes, you can do this.

As the controller of both the Grim Apothecary "When you play me" trigger and the Pridestalker (Rengar Legend) trigger (both triggered by the same event: playing a unit), you choose the order in which they are placed on the chain. To achieve your goal, you should order them so that the Grim Apothecary trigger is at the bottom and the Pridestalker trigger is on top.

Sequence:

1. Play Grim Apothecary: You play the unit; it enters the battlefield.
2. Triggers: Both the Grim Apothecary "When you play me" ability and the Pridestalker ability trigger.
3. Place on Chain: You, as the controller, place them on the chain. You place the Grim Apothecary ability on the chain first, then place the Pridestalker ability on top.
4. Chain State: The state is now Closed.
5. Resolution (LIFO):
   • Pridestalker ability resolves: You choose a unit (e.g., another unit on your battlefield) and give it +1 Might.
   • Grim Apothecary ability resolves: You make your choice for the "you may" effect. Since Grim Apothecary is a friendly unit on the battlefield, it is a legal target. You may choose to return Grim Apothecary to your hand.

Key Rules Supporting this:

• Simultaneous Triggers (Rule 383.3.d): When multiple triggered abilities you control trigger simultaneously, you decide the order they are placed on the chain.
• LIFO Resolution: The chain resolves from top to bottom (Last-In, First-Out).
• Grim Apothecary Target (FAQ 9746): Grim Apothecary can target itself, as it is a "friendly unit" once it is in play.
• Pridestalker (FAQ 9769): You can target any unit with Pridestalker's ability.

Note: This interpretation assumes you have the required energy to play Grim Apothecary and that you are in a situation where you can play it (such as having sufficient energy and being in your Main Phase if playing it normally, or meeting Ambush conditions).