Yes, "fen" and "feeds" is an example of a slant rhyme. Slant rhymes involve words that have similar-sounding endings but are not a perfect match, adding a subtle rhyme effect while not fully rhyming.
Of the pairs you offer: 'swamp' and 'damp' are the slant rime. 'hear' and 'near' is a true rime; fen / feeds, warm / true do not rime at all. In a true rime, the vowel sound and the consonants which follow it are a precise match: beat / sweet, hope / soap, grew / true. In a slant rime, the vowel sounds are a near match (any trailing consonants will normally be identical): beat / mate, hope / shape, grew / sow. You need to be careful with slant rime (most teachers aren't): sometimes a true rime in one accent will be a slant rime in a different one. Mayor / chair is a true rime in the English home counties, but a slant rime most other places that English is spoken.
Fen, hen, when, ten, yen, den, again, ken, men, pen, ren, zen
clean, been, seen, keen, mean (3 kinds), lean, teen, fen, sardine, Hardin, Cardin
Away to the Dismal Swamp he speeds-His path was rugged and sore,Through tangled juniper, beds of reeds,Through many a fen where the serpent feeds,And man never trod before. the ryme scheme is ABAAB and the meter is iambic.
There are several words that rhyme with ven. Here are some of them: Ben den fen hen ken men pen ten then Venn wren yen zen
fen phen
The population of Borough Fen is 130.
Yao Fen was born in 1967.
Gervase Fen was created in 1944.
Fen Cresswell died in 1966.
Fen Cresswell was born in 1915.
Fen Drayton's population is 827.