Both Uses

interloper
in
Self-Reliance, by Emerson
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Let him not peep or steal, or skulk up and down with the air of a charity-boy, a bastard, or an interloper, in the world which exists for him.†

interloper = someone or something that, without invitation, inserts itself
The soul is no traveler; the wise man stays at home, and when his necessities, his duties, on any occasion call him from his house, or into foreign lands, he is at home still; and shall make men sensible by the expression of his countenance, that he goes the missionary of wisdom and virtue, and visits cities and men like a sovereign, and not like an interloper or a valet.†
*

Definitions:

(1) (interloper)
someone or something that, without invitation, inserts itself -- such as an uninvited guest at a party or a species that invades non-native territory
(2) (meaning too rare to warrant focus)