in 1841, there's a Walter Evan aged 20, ag lab, enumerated in a detached building at Sabulon farm in St Clears HO107/1382/1 folio 34 pg 6
Also in 1841 HO107/1448/1 folio 22 pg 11
Row Park, Amroth, Pembrokeshire
Josiah Evan 60 farmer
Ruhamah 50
Elizabeth 30
Thomas 27 labourer
William 14
Harriet 12
Josiah 8
all not born in county
1861 Steps, St Issells, Pembs RG9/4151 folio 91 pg 35
Josiah Evans head mar 85 gardener b. Carms, Llanboidy
Ruhamah wife 74 b. Carms, Llanwinio
Elizabeth grand dau 16 b. Pembs, St Issells
As the baptism is submitted and not extracted, you need to verify it from another source. You could get a copy of the parish entry from the record office in Carmarthen
http://www.ancestor-search.info/CRO-Carmarthenshire.htmDo you have Walter's marriage cert to confirm his father's name?
The final 's' comes & goes on many Welsh surnames at this time -you find Davies as David.