Lowest level forms to forge the starter polymorph ring mimics start out with, for probably the max (ring must give at least X Au) / easiest (monster must have FRIENDS flag or appear in "escorts" (ie another -often somewhat stronger- monster has ESCORT or ESCORTS flag, spawning this monster in groups around it) or in "Pits" so it's easy to farm them) profit ratio, when just selling the forged ring to shop 6 for Au (on forging, the ring will lose its 'starter item' status, so it can be sold). This means the forms must grant at least one ring slot so the player can equip and activate the poly-player ring.
NOTE: Price depends not only on charisma and shopkeeper, but also on the ring's remaining energy, so your values may vary somewhat (or greatly, if the ring is already quite depleted).
The paragraphs are sorted by monster level first, then by ring value second.
Monster levels 1-4:
Some of these don't have FRIENDS flag or appear in escorts, but due to their low level you only need 1-4 of them anyway, so they were added to this list.
The level 1 and 2 monsters are especially easily hunted in the Training Tower for instant money.
( 29) L 1, 5XP, Small kobold : 860 Au (solo only)
( 60) L 2, 7XP, Cave spider : 1430 Au
( 61) L 2, 8XP, Crow : 1509 Au (solo only)
( 62) L 2, 8XP, Wild cat : 1534 Au (solo only)
( 87) L 4, 10XP, Snotling : 934 Au
( 83) L 4, 10XP, Novice ranger : 1161 Au
(1000) L 4, 10XP, Novice mindcrafter : 1070 Au
( 68) L 4, 8XP, Raven : 1717 Au (solo only)
Monster levels 5-10, filtered for at least 3000 Au:
( 102) L 5, 25XP, Large kobold : 3057 Au (Vort/Mughash (L7) escorts)
( 149) L 8, 25XP, Hill orc : 2947 Au (pfft, almost 3000 Au)
( 173) L 9, 25XP, War bear : 3332 Au
( 189) L 10, 25XP, Blue horror : 3520 Au
Monster levels 11-20, filtered for at least 4000 Au:
( 224) L 12, 28XP, Master Yeek : 4329 Au (Orfax (L11)/Boldor (L15) escorts)
( 262) L 15, 70XP, Black Ogre : 4058 Au
( 270) L 15, 55XP, Wererat : 5652 Au (Rat King (L18) escorts)
( 285) L 16, 75XP, Ogrillon : 4334 Au
( 347) L 20, 150XP, Werewolf : 13872 Au (Wolf Chieftain (L26) escorts, rarer)
Monster levels 21-30, filtered for at least 6000 Au:
( 278) L 23, 180XP, Frost giant : 8945 Au (Appears in 'P' pits)
( 288) L 25, 220XP, Fire giant : 10702 Au (Appears in 'P' pits)
( 415) L 26, 80XP, Cave Ogre : 6150 Au
( 430) L 27, 80XP, Half-ogre : 7408 Au
( 321) L 28, 250XP, Stone giant : 13453 Au (Appears in 'P' pits)
( 452) L 28, 150XP, Deep one : 13775 Au
( 463) L 29, 300XP, Aklash : 7228 Au (Appears in deeper 'T' pits)
( 963) L 30, 250XP, Aranea : 6434 Au
( 479) L 30, 100XP, Mountain ogre : 9552 Au
( 327) L 30, 130XP, Ghast : 10159 Au (Ghoulking (L32) escorts)
last but not least, it's solo but gets honourable mention because it gives massive Au:
( 955) L 30, 6000XP, Green dragonrider : 59409 Au (solo only)
At these levels chances are you have long forgotten about the starter ring and have found more poly-player rings, still one more table for mid-level monsters - however, some of these are trumped by Level 20-31 monster values:
Monster levels 31-40, filtered for at least 9000 Au:
( 349) L 31, 500XP, Cloud giant : 20283 Au (Appears in 'P' pits)
( 509) L 35, 420XP, Water troll : 9028 Au
( 535) L 35, 250XP, Leng Spider : 9880 Au
( 613) L 35, 600XP, Hellhound : 14609 Au
( 487) L 35, 1000XP, Storm giant : 23547 Au (Appears in 'P' pits)
( 538) L 36, 450XP, Olog : 10000 Au (woot)
( 902) L 37, 220XP, Scrag : 9765 Au
( 648) L 40, 750XP, Vrock : 12400 Au
( 631) L 40, 800XP, War troll : 17909 Au
( 444) L 40, 1500XP, Cyclops : 33064 Au (Appears in 'P' pits)
solo, but can be found in deeper T pits:
( 620) L 38, 1500XP, Eldrak : 20823 Au (Appears in deeper 'T' pits)
( 621) L 39, 2000XP, Ettin : 32585 Au (Appears in deeper 'T' pits)
again honourable solo mention for massive Au:
( 956) L 40, 8000XP, Blue dragonrider : 80795 Au (solo only)
A complete table of form prices is found in the links section, leading to: https://www.tomenet.eu/downloads/polyforms.txt