Theory:

It is not, I suppose, in any way strange that the average Londoner should not recognise an otter, but the variety of guesses as to what kind of animal this might be came as a surprise to me. Otters belong to a comparatively small group of animals called Mustellines, shared by the badger, mongoose, weasel, stoat, mink and others. I faced a continuous barrage of conjectural questions that sprayed all the Mustellines but the otter; more random guesses hit on ‘a baby seal’ and ‘a squirrel.’ ‘Is that a walrus, mister?’ reduced me to giggles, and outside a dog show I heard ‘a hippo’. A beaver, a bear cub, a leopard — one, apparently, that had changed its spots — and a ‘brontosaur’; Mij was anything but an otter.

Explanation:

 

The author thought it odd that the locals couldn't identify an otter because they are a rare species in London. People made illogical assumptions about what Mij was. Otters are a member of the Mustelline family, which also includes the badger, mongoose, weasel, stoat, and mink. Except for Otter, all of the group's creatures could be predicted. The two most well-known theories were a baby seal or a squirrel. Some people thought Mijbil was a walrus, hippo, beaver, or leopard.

 

Meanings of difficult words:

 
Sl. No
Words
Meanings
1
ValleyA low area of land between hills or mountains, typically with a river or stream flowing through it
2
CrestThe top of a mountain or hill
3
RipeHaving arrived at the fitting stage or time for a particular action or purpose
4
DownpourA heavy fall of rain
5
ShowerA brief and usually light fall of rain, hail, sleet, or snow
6
IntimatelyIn a way that involves detailed knowledge
Reference:

National Council of Educational Research and Training (2007). First Flight. Mijbil the Otter - Gavin Maxwell (pp. 102 - 110). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi..