Of course you can use any creatures from the Starfinder Alien Archive as threats in a Really Wild West campaign, but in most cases you’ll want to reflavor them to something more appropriate for it’s 1891 aesthetic and technology level.

It’s useful to dream up brand-new threats as well of course, to get foes hat are unique to the dangerous world of pulp theosophy and super-science that is Really Wild West. Here is a very RWW-themed undead, which may be encountered alone or in mass numbers as dictated by the plot. If you want to make different of higher-CR gulchers, just take any undead and replace one of its offensive powers with bad off, add false life, lower its EAC by 2 and raise its KAC by 1.

GULCHER (CR 1)
XP 400
NE Medium undead
Init +2; Senses darkvision 60 ft.; Perception +5
DEFENSE
HP 24
EAC 9; KAC 14
Fort +3; Ref +3; Will +3; DR 3/magic; Immunities undead immunities
OFFENSE
Speed 30 ft.
Melee pitchfork (or other tool) +8 (1d6+5 P; critical: bad off) or
Ranged revolver +6 (1d6+1 P)
Special Attacks bad off (DC 11)
STATISTICS
Str +4; Dex +2; Con —; Int +0; Wis +1; Cha +0
Skills Athletics +10
Other Abilities unliving
SPECIAL ABILITIES
Bad Off (Su)
A gulcher is imbued with the bad times that lead to its sorry state, and can sometimes inflict its bad luck and sad-sack existence on those it hurts. Any attack from a glucher that scores a critical hit causes the target to feel down and out, gaining the sickened condition for 24 hours, or until the target receives a morale bonus (to anything) or is the recipient of a Diplomacy check to improve their attitude.
A gulcher can also attempt to inflict its bad off ability on a creature as a standard action, screaming in contagious misery. Used this way the ability is sense-dependent and the target can negate it with a successful DC 11 Will save.
A DC 11 Mysticism check can identity the nature of a creature being bad off, and reveal the circumstances that negate this effect. Bad off is a curse effect.

False Life (Ex)
A gulcher that doesn’t realize its own true nature is not affected by spells or abilities that only target undead.
ECOLOGY
Environment any
Organization solitary, pair, posse (3–12), or settlement (13+)

Gulchers are undead that appear to be gaunt, dirty, badly-tended humans, often dressed in patched and worn prairie clothing, though they can also have the appearance of drovers, gunfolk, miners, merchants, gunfolk, and native people can also become gulchers. Most have sallow skin, yellowed, crooked teeth, stringy hair, and sullen or bloodshot eyes. A few appear jaundiced.

Gulchers are most often normal people who went through a time of despair, tribulation, hunger, and pestilence, and died. But they didn’t notice. Things had been so bad, for so long, that dying would be a relief, and gulchers just don’t expect anything to get better.

As long as a gulcher is unaware it has become an undead, it goes about the dreary and colorless motions of living a life. It eats, if food is available, lies in bed and doesn’t realize it never sleeps, sucks down duststorms and doesn’t realize it should choke. In this state, the gulcher isn’t affected by powers that only effect undead, but it also isn’t immune to fear and emotion effects, and takes the penalties for being shaken at all times (though this is more a dreary lack of verve than true fear).

All this changes if the gulcher is made aware of its state. The easiest way to do this is to deal piercing or slashing damage to it – gulchers have thick, black blood and realize the horrible truth of their state if they see their own tarlike vitae. Evidence of their lifeless existence, lack of food, lack of sleep, and so on, can also be used to convince a gulcher it is no longer living with a DC 15 Diplomacy check. Once it knows that even the peace of the grave is denied it, a gulcher is slowly consumed with a desire to make everyone and everything as pained and hopeless as its own existence.

It’s not unknown for entire towns to become gulchers, often during thunderdusts, droughts, and locust plagues. Sometimes one or two take the gray journey, and their desire to cause misery slowly kill off everyone else in town. Othertimes a real bad situation takes out near everyone most all at once. And sometimes, a drakul, ghul, black spirit, or other bigtime black hat decided to take over a town as a base of operation, and intentionally nurses the despair that causes god-fearin’ folk to become the things other folk fear.

In very rare cases, gulchers perform a useful service, such as toiling at a mostly-played out mine that would be pointless for living creatures to port the food and water needed to operate, operating rickety barges on distant rivers with little traffic, or slowly clearing stones from areas that might, in a few decades, be worthwhile farmlands. Of course, these gulchers are also likely to be angered by the sight of anyone doing better than they, and may drown passengers, or dump scorpions into their sleeping blankets.