Summon Nature's Ally spells summon natural creatures from one's current plane and neighboring planes, which in Mechanus' case includes Arcadia, Acheron, and the Outlands. Most natural animals are not found on Mechanus, and are rare on Acheron. More are found on the Outlands, which GMs may use to permit a Summon Nature's Ally spell on Mechanus to summon a natural animal. More flavorful, however, would be to replace the typical list of summonable creatures with creatures strictly native to Mechanus and neighboring planes. One such list follows.
Mechanus SNA list
1: Formian worker (Note 1); Gear spirit (Note 2); Moigno; Modron, monodrone
2: Inevitable, Arbiter; Elemental, Small (any) (Note 3); Modron, duodrone
3: Blink dog; Formian warrior; Modron, tridrone
4: Barghest; Elemental, Medium (any); Modron, quadrone
5: Achaierai; Barghest, greater; Elemental, Large (any); Modron, pentadrone
6: Blink dog Taiji Devotee (Note 4); Elemental, Huge (any); Formian Taskmaster
7: Axiomite; Inevitable, Zelekhut; Elemental, Greater (any); Modron, decaton (Note 5)
8: Formian Myrmarch; Modron, nonaton
9: Inevitable, Kolyarut; Elemental, Elder (any); Modron, octon
(1) Axiomites, formians and modrons require Bluff checks if a summoned creature is to properly emulate the telepathic presence necessary to convince other members of the race that a summoned version of a member of their race is real. Axiomites are a PFSRD creature. For modrons, see the Manual of the Planes web enhancement.
(2) Note to authors: We would need to convert moignos and gear spirits, which have interesting little attacks but not a lot of Hit Dice. Once we were done they might be 2nd or 3rd-level summons.
(3) In 2nd edition Planescape, elementals summoned on Outer Planes were pseudo-elementals made up of the plane's own essence given the form of the desired element. Pseudo-elementals are like elementals in every way except that they have the alignment traits of the plane upon which they were summoned, and often personality traits reflective of that environment. They are no brighter than other summoned elementals and are easily persuaded, but will not take acts blatantly in contravention of the plane's alignment.
Inevitables likewise function better in pursuit of their normal goals. Zelekhut inevitables pursue fugitives from justice. Kolyarut inevitables punish oathbreakers. Marut inevitables confound those who would deny death. They may attempt to pervert orders if they see an opportunity to pursue their natural mission, and will in any case not do anything which directly contravenes that type of inevitable's normal purpose (a zelekhut will refuse to aid a fugitive in escaping from the law, for instance).
(4) A blink dog with 4 levels of Monk and a monk's belt. The style requires selecting Stunning Fist and Combat Reflexes, and trains Wisdom for the 4th-level improvement. The monastery is almost certainly somewhere on Arcadia, but must be quite reclusive.
(5) Hierarch modrons summoned with Summon Nature's Ally spells are specific, identifiable individuals within societal ranks of finite size, although the spell cannot be worded so as to select a desired individual. (Non-modrons have an extremely difficult time telling modrons of the same rank apart anyway.) Similar to a resurrection spell, the hirerarch modron can sense its summoner's intent and may refuse to appear if their alignment or current intentions are nonlawful; this causes the spell to be wasted. After a successful summoning, a summoner may later encounter the particular modron they summoned, who will remember the encounter. Interestingly, whenever this has happened the summoned modron seems to have regarded the summoner as having had the legal right to do so. Summoning a hierarch modron and ordering it to its temporary apparent death, however, will be regarded negatively.
How this list was constructed:
We began with a listing of the Challenge Ratings of creatures that could be summoned by Summon Nature's Ally spells. Roughly speaking, CRs match the level of the spell for levels 1-6, but at levels 5 and 6 CRs may be 6 or 7 respectively, and at 7-9, CRs sometimes exceed the level of the spell by up to 2.
Next, we considered the creatures likely to appear on Mechanus and neighboring planes. Any Extraplanar creature with the Lawful subtype was a candidate, though devils and archons were eliminated because these species are not native to planes that border Mechanus. Formians, modrons, Axiomites, and inevitables also made the list. Blink dogs were added because they are native to Arcadia, and achaierai and barghests are native to Acheron.
(If anyone can think of on-theme creatures to add using these guidelines, I'm all ears! Should low-ranking rilmani be possible summons? I figured petitioners should not be.)