The spell Summon Warlock's Ally allows you to summon a creature associated with your pact; however, from what I can tell there is more than one option for that. In the above link, you'll find the Shadow Lurk which seems to be open to all Warlock's regardless of pact, the Wood Woad Guardian which is open to Fey pact and I've also found Satyr of the Night which seems to be available to Fey pact as well.
Can a warlock summon any of these creatures it wants? Do you choose at the time of summoning? Else, do you choose beforehand and become stuck with that creature permanently after the fact? Are there other options available that I haven't found yet?